feat(fees): working network fee selection for sell and pay

lib/core/widgets/fees/fee_selection_label.dart

@@ -0,0 +1,38 @@
+import 'package:bb_mobile/core/fees/domain/fees_entity.dart';
+import 'package:bb_mobile/core/utils/amount_formatting.dart';
+import 'package:bb_mobile/core/utils/build_context_x.dart';
+import 'package:flutter/widgets.dart';
+
+/// Value shown on the "Fee Priority" row of the exchange confirmation screens
+/// (sell, pay) for the committed selection. Presets read as their tier name; a
+/// custom fee reads as the value that was typed, because "Custom Fee" on its
+/// own hides the only number that tells the tiers apart.
+///
+/// [fastestLabel] comes from the caller: each flow ships its own translated
+/// "Fastest" string, while the remaining tiers have no per-flow copy and share
+/// the send keys.
+String feeSelectionRowLabel(
+  BuildContext context, {
+  required FeeSelection selection,
+  required NetworkFee? customFee,
+  required String fastestLabel,
+}) => switch (selection) {
+  FeeSelection.fastest => fastestLabel,
+  FeeSelection.economic => context.loc.sendEconomyFee,
+  FeeSelection.slow => context.loc.feePrioritySlow,
+  FeeSelection.custom => switch (customFee) {
+    RelativeFee(:final satPerKwu) =>
+      '${_formattedRate(satPerKwu / 250.0)} ${context.loc.sendSatsPerVB}',
+    AbsoluteFee(:final sats) =>
+      '${FormatAmount.sats(sats)} ${context.loc.sendSats}',
+    // Custom is selected but nothing has been typed yet — only reachable
+    // while the modal is open, since dismissal rolls an empty field back.
+    null => context.loc.sendCustomFee,
+  },
+};
+
+/// Whole rates lose the decimals: "3 sats/vB" rather than "3.00 sats/vB".
+String _formattedRate(double satPerVbyte) =>
+    satPerVbyte == satPerVbyte.roundToDouble()
+    ? satPerVbyte.toStringAsFixed(0)
+    : satPerVbyte.toStringAsFixed(2);

lib/features/pay/pay_locator.dart

@@ -16,6 +16,8 @@ import 'package:bb_mobile/features/send/domain/usecases/calculate_liquid_absolut
 
 import 'package:bb_mobile/core/wallet/domain/usecases/prepare_bitcoin_send_usecase.dart';
 import 'package:bb_mobile/features/send/domain/usecases/prepare_liquid_send_usecase.dart';
+import 'package:bb_mobile/features/send/domain/usecases/preview_bitcoin_fee_presets_usecase.dart';
+import 'package:bb_mobile/features/send/domain/usecases/preview_bitcoin_fee_usecase.dart';
 import 'package:bb_mobile/features/send/domain/usecases/sign_bitcoin_tx_usecase.dart';
 import 'package:bb_mobile/features/send/domain/usecases/sign_liquid_tx_usecase.dart';
 import 'package:get_it/get_it.dart';
@@ -76,6 +78,9 @@ class PayLocator {
         getAddressAtIndexUsecase: locator<GetAddressAtIndexUsecase>(),
         getWalletUtxosUsecase: locator<GetWalletUtxosUsecase>(),
         getOrderUsecase: locator<GetOrderUsecase>(),
+        previewBitcoinFeeUsecase: locator<PreviewBitcoinFeeUsecase>(),
+        previewBitcoinFeePresetsUsecase:
+            locator<PreviewBitcoinFeePresetsUsecase>(),
       ),
     );
   }

lib/features/pay/presentation/pay_event.dart

@@ -18,10 +18,10 @@ sealed class PayEvent with _$PayEvent {
     required OrderBitcoinNetwork network,
   }) = PayExternalWalletNetworkSelected;
   const factory PayEvent.orderRefreshTimePassed() = PayOrderRefreshTimePassed;
-  const factory PayEvent.sendPaymentConfirmed({
-    required FeeSelection feeSelection,
-    NetworkFee? customFee,
-  }) = PaySendPaymentConfirmed;
+  // The fee to pay comes from the committed selection on PayPaymentState, not
+  // from the event: the modal writes it there and the summary row reads it
+  // back, so a second copy on the event could only ever disagree (#2521).
+  const factory PayEvent.sendPaymentConfirmed() = PaySendPaymentConfirmed;
   const factory PayEvent.pollOrderStatus() = PayPollOrderStatus;
   const factory PayEvent.replaceByFeeChanged({required bool replaceByFee}) =
       PayReplaceByFeeChanged;
@@ -30,4 +30,32 @@ sealed class PayEvent with _$PayEvent {
   const factory PayEvent.loadUtxos() = PayLoadUtxos;
   const factory PayEvent.updateOrderStatus({required String orderId}) =
       PayUpdateOrderStatus;
+
+  // ────── shared fee modal (FeeModalActions) ──────
+  /// A preset tile was picked. Commits the tier and rebuilds the fee estimate.
+  const factory PayEvent.feeOptionSelected(FeeSelection feeSelection) =
+      PayFeeOptionSelected;
+
+  /// The typed custom fee is committed — triggers the fee recalculation.
+  const factory PayEvent.customFeeChanged(NetworkFee fee) = PayCustomFeeChanged;
+
+  /// Custom-fee field changed: select `custom` without rebuilding yet, and
+  /// snapshot the prior selection so dismissal can roll it back.
+  const factory PayEvent.customFeeArmed(NetworkFee fee) = PayCustomFeeArmed;
+
+  /// The custom-fee field was cleared — roll the arm back immediately so a
+  /// stale typed value cannot survive to dismissal.
+  const factory PayEvent.customFeeDisarmed() = PayCustomFeeDisarmed;
+
+  /// The modal was dismissed without picking a preset: commit the armed value
+  /// when it clears the relay floor, otherwise roll back.
+  const factory PayEvent.customFeeFinalized() = PayCustomFeeFinalized;
+
+  /// Debounced preview build for the typed rate (fills the `custom` slot).
+  const factory PayEvent.customFeePreviewRequested(NetworkFee fee) =
+      PayCustomFeePreviewRequested;
+
+  /// Preview builds for the three presets, fired on every modal open.
+  const factory PayEvent.presetFeesPreviewRequested() =
+      PayPresetFeesPreviewRequested;
 }

lib/features/pay/presentation/pay_state.dart

@@ -35,6 +35,27 @@ sealed class PayState with _$PayState {
     @Default([]) List<WalletUtxo> selectedUtxos,
     @Default(true) bool replaceByFee,
     double? exchangeRateEstimate,
+    // Bitcoin fee selection (#2521), mirroring SellPaymentState: the payin is
+    // built at the rate picked in the shared fee modal, not a hardcoded
+    // Fastest.
+    FeeOptions? bitcoinFees,
+    NetworkFee? customFee,
+    @Default(FeeSelection.fastest) FeeSelection selectedFeeOption,
+    // Arm/disarm snapshot for the custom-fee field: typing commits `custom` so
+    // the preset tiles deselect, and dismissal either finalizes the value or
+    // rolls back to these. Internal to the modal flow; the UI must not read
+    // them.
+    FeeSelection? armPriorSelection,
+    NetworkFee? armPriorCustomFee,
+    // Real fees read from unsigned PSBTs, one slot per tier, so the modal never
+    // shows rate × vsize arithmetic. Display only: the confirmation rebuilds
+    // the payin at the committed rate rather than broadcasting a cached PSBT,
+    // because a price-lock refresh can move the payin amount.
+    @Default(BitcoinFeePreviewCache.empty)
+    BitcoinFeePreviewCache feePreviewCache,
+    // vsize of the last payin build — needed to express an absolute custom fee
+    // as a rate for the relay-floor checks.
+    int? bitcoinTxSize,
   }) = PayPaymentState;
   const factory PayState.success({required FiatPaymentOrder payOrder}) =
       PaySuccessState;
@@ -191,6 +212,8 @@ extension PayWalletSelectionStateX on PayWalletSelectionState {
     List<WalletUtxo>? utxos,
     int? absoluteFees,
     double? exchangeRateEstimate,
+    FeeOptions? bitcoinFees,
+    int? bitcoinTxSize,
   }) {
     return PayPaymentState(
       userSummary: userSummary,
@@ -202,6 +225,8 @@ extension PayWalletSelectionStateX on PayWalletSelectionState {
       absoluteFees: absoluteFees,
       exchangeRateEstimate: exchangeRateEstimate,
       utxos: utxos ?? [],
+      bitcoinFees: bitcoinFees,
+      bitcoinTxSize: bitcoinTxSize,
     );
   }
 
@@ -231,6 +256,26 @@ extension PayPaymentStateX on PayPaymentState {
   bool get canConfirmPayment => isInternalWallet && selectedUtxos.isNotEmpty;
   bool get isProcessing => isConfirmingPayment || isPolling;
 
+  /// Fee the payin must be built at, resolved from the committed selection.
+  /// Null while the presets have not loaded (or custom was selected with no
+  /// value) — the caller decides, rather than silently reverting to Fastest,
+  /// which is the bug #2521 describes.
+  NetworkFee? get selectedFee => switch (selectedFeeOption) {
+    FeeSelection.fastest => bitcoinFees?.fastest,
+    FeeSelection.economic => bitcoinFees?.economic,
+    FeeSelection.slow => bitcoinFees?.slow,
+    FeeSelection.custom => customFee,
+  };
+
+  /// Fee selection is only editable before the confirmation starts: the
+  /// transaction on its way to the network was built at the committed rate, so
+  /// rebuilding under it would pay a different fee than the one shown. Liquid
+  /// payins have no fee choice at all.
+  bool get canEditFees =>
+      !isConfirmingPayment &&
+      selectedWallet != null &&
+      !selectedWallet!.isLiquid;
+
   String get bip21InvoiceData {
     final order = payOrder;
     String invoiceString = '';

lib/features/pay/ui/screens/pay_send_payment_screen.dart

@@ -5,6 +5,8 @@ import 'package:bb_mobile/core/utils/amount_conversions.dart';
 import 'package:bb_mobile/core/utils/amount_formatting.dart';
 import 'package:bb_mobile/core/utils/build_context_x.dart';
 import 'package:bb_mobile/core/widgets/buttons/button.dart';
+import 'package:bb_mobile/core/widgets/fees/fee_options_modal.dart';
+import 'package:bb_mobile/core/widgets/fees/fee_selection_label.dart';
 import 'package:bb_mobile/core/widgets/loading/fading_linear_progress.dart';
 import 'package:bb_mobile/core/widgets/loading/loading_line_content.dart';
 import 'package:bb_mobile/core/widgets/scrollable_column.dart';
@@ -207,15 +209,8 @@ class PaySendPaymentScreen extends StatelessWidget {
                           : context.loc.paySecureBitcoinWallet
                       : ''),
             ),
-            if (wallet != null && !wallet.isLiquid) ...[
-              _DetailRow(
-                title: context.loc.payFeePriority,
-                value: context.loc.payFastest,
-                onTap: () {
-                  debugPrint('Tapped Fee Priority');
-                },
-              ),
-            ],
+            // Liquid payins pay the network minimum, so there is nothing to pick.
+            if (wallet != null && !wallet.isLiquid) const _FeePriorityRow(),
             _DetailRow(
               title: context.loc.payNetworkFees,
               value: context.select((PayBloc bloc) {
@@ -232,10 +227,7 @@ class PaySendPaymentScreen extends StatelessWidget {
             _BottomButtons(
               onContinuePressed: () {
                 context.read<PayBloc>().add(
-                  const PayEvent.sendPaymentConfirmed(
-                    feeSelection: FeeSelection.fastest,
-                    customFee: null,
-                  ),
+                  const PayEvent.sendPaymentConfirmed(),
                 );
               },
             ),
@@ -278,6 +270,69 @@ class PaySendPaymentScreen extends StatelessWidget {
   }
 }
 
+/// "Fee Priority" row: opens the shared fee modal and shows the committed
+/// selection (#2521). The row goes inert — plain text, no chevron — once the
+/// confirmation starts, since the payin being signed was built at the rate
+/// showing here.
+class _FeePriorityRow extends StatelessWidget {
+  const _FeePriorityRow();
+
+  @override
+  Widget build(BuildContext context) {
+    final (selectedFeeOption, customFee, canEditFees) = context.select((
+      PayBloc bloc,
+    ) {
+      final state = bloc.state;
+      if (state is! PayPaymentState) {
+        return (FeeSelection.fastest, null as NetworkFee?, false);
+      }
+      return (state.selectedFeeOption, state.customFee, state.canEditFees);
+    });
+
+    return _DetailRow(
+      title: context.loc.payFeePriority,
+      value: feeSelectionRowLabel(
+        context,
+        selection: selectedFeeOption,
+        customFee: customFee,
+        fastestLabel: context.loc.payFastest,
+      ),
+      onTap: canEditFees
+          ? () async {
+              final bloc = context.read<PayBloc>();
+              final selected = await BlurredBottomSheet.show<String>(
+                context: context,
+                child: FeeOptionsModal(
+                  viewState: bloc,
+                  actions: bloc,
+                  defaultAbsoluteCustomFee: false,
+                  customFeeColors: FeeModalCustomFeeColors(
+                    tile: context.appColors.surface,
+                    shadow: context.appColors.border,
+                    unselectedIcon: context.appColors.textMuted,
+                  ),
+                ),
+              );
+              if (selected != null) {
+                // A preset tile was tapped; the handler discards any arm left
+                // over from typing in the custom field.
+                bloc.add(
+                  PayEvent.feeOptionSelected(
+                    FeeSelectionName.fromString(selected),
+                  ),
+                );
+              } else {
+                // Dismissed without picking. Typing IS the selection and
+                // dismissing IS the apply, so a typed rate is committed here
+                // (or rolled back when it is below the relay floor).
+                bloc.add(const PayEvent.customFeeFinalized());
+              }
+            }
+          : null,
+    );
+  }
+}
+
 class _DetailRow extends StatelessWidget {
   final String title;
   final String? value;