refactor(sell): localize loading label and rename error-card widget

Replace a hardcoded 'Loading...' with the existing sellLoadingGeneric
key, and rename the private _SellError error-card widget to
_SellErrorCard to avoid confusion with the SellFailure family.

Author: wired-pasteque

lib/features/sell/presentation/sell_failure_l10n.dart

@@ -5,19 +5,18 @@ import 'package:flutter/widgets.dart';
 
 extension SellFailureL10n on SellFailure {
   String toTranslated(BuildContext context) => switch (this) {
-        SellUnauthenticatedFailure() => context.loc.sellUnauthenticatedError,
-        SellBelowMinAmountFailure(:final minAmountSat) =>
-          context.loc.sellBelowMinAmountError(FormatAmount.sats(minAmountSat)),
-        SellAboveMaxAmountFailure(:final maxAmountSat) =>
-          context.loc.sellAboveMaxAmountError(FormatAmount.sats(maxAmountSat)),
-        SellInsufficientBalanceFailure(:final requiredAmountSat) =>
-          context.loc.sellInsufficientBalanceError(
-            FormatAmount.sats(requiredAmountSat),
-          ),
-        SellPrepareTransactionFailure() =>
-          context.loc.sellErrorFeesNotCalculated,
-        SellSendPaymentFailure() => context.loc.sellErrorSendPayment,
-        SellLoadUtxosFailure() => context.loc.sellErrorLoadUtxos,
-        SellUnexpectedFailure() => context.loc.oopsSomethingWentWrong,
-      };
+    SellUnauthenticatedFailure() => context.loc.sellUnauthenticatedError,
+    SellBelowMinAmountFailure(:final minAmountSat) =>
+      context.loc.sellBelowMinAmountError(FormatAmount.sats(minAmountSat)),
+    SellAboveMaxAmountFailure(:final maxAmountSat) =>
+      context.loc.sellAboveMaxAmountError(FormatAmount.sats(maxAmountSat)),
+    SellInsufficientBalanceFailure(:final requiredAmountSat) =>
+      context.loc.sellInsufficientBalanceError(
+        FormatAmount.sats(requiredAmountSat),
+      ),
+    SellPrepareTransactionFailure() => context.loc.sellErrorFeesNotCalculated,
+    SellSendPaymentFailure() => context.loc.sellErrorSendPayment,
+    SellLoadUtxosFailure() => context.loc.sellErrorLoadUtxos,
+    SellUnexpectedFailure() => context.loc.oopsSomethingWentWrong,
+  };
 }

lib/features/sell/ui/screens/sell_external_wallet_network_selection_screen.dart

@@ -84,7 +84,7 @@ class SellExternalWalletNetworkSelectionScreen extends StatelessWidget {
                           ),
                   ),
                   const Gap(24.0),
-                  const _SellError(),
+                  const _SellErrorCard(),
                 ],
               ),
             ),
@@ -95,8 +95,8 @@ class SellExternalWalletNetworkSelectionScreen extends StatelessWidget {
   }
 }
 
-class _SellError extends StatelessWidget {
-  const _SellError();
+class _SellErrorCard extends StatelessWidget {
+  const _SellErrorCard();
 
   @override
   Widget build(BuildContext context) {

lib/features/sell/ui/screens/sell_receive_payment_screen.dart

@@ -168,7 +168,7 @@ class SellReceivePaymentScreen extends StatelessWidget {
             _buildDetailRow(
               context,
               context.loc.sellOrderNumber,
-              order?.orderNumber.toString() ?? 'Loading...',
+              order?.orderNumber.toString() ?? context.loc.sellLoadingGeneric,
               copyValue: order?.orderNumber.toString(),
             ),
             const Gap(48),

lib/features/sell/ui/screens/sell_screen.dart

@@ -73,7 +73,7 @@ class _SellScreenState extends State<SellScreen> {
                   },
                 ),
                 const Gap(16.0),
-                const _SellError(),
+                const _SellErrorCard(),
                 const Spacer(),
                 SellAmountInputBottomButtons(
                   formKey: _formKey,
@@ -98,8 +98,8 @@ class _SellScreenState extends State<SellScreen> {
   }
 }
 
-class _SellError extends StatelessWidget {
-  const _SellError();
+class _SellErrorCard extends StatelessWidget {
+  const _SellErrorCard();
 
   @override
   Widget build(BuildContext context) {

lib/features/sell/ui/screens/sell_send_payment_screen.dart

@@ -317,7 +317,7 @@ class _BottomButtons extends StatelessWidget {
     );
     return Column(
       children: [
-        const _SellError(),
+        const _SellErrorCard(),
         if (wallet != null && !wallet.isLiquid) ...[
           BBButton.big(
             label: context.loc.sellAdvancedSettings,
@@ -349,8 +349,8 @@ class _BottomButtons extends StatelessWidget {
   }
 }
 
-class _SellError extends StatelessWidget {
-  const _SellError();
+class _SellErrorCard extends StatelessWidget {
+  const _SellErrorCard();
 
   @override
   Widget build(BuildContext context) {

lib/features/sell/ui/screens/sell_wallet_selection_screen.dart

@@ -76,7 +76,7 @@ class SellWalletSelectionScreen extends StatelessWidget {
                           ),
                   ),
                   const Gap(24.0),
-                  const _SellError(),
+                  const _SellErrorCard(),
                 ],
               ),
             ),
@@ -87,8 +87,8 @@ class SellWalletSelectionScreen extends StatelessWidget {
   }
 }
 
-class _SellError extends StatelessWidget {
-  const _SellError();
+class _SellErrorCard extends StatelessWidget {
+  const _SellErrorCard();
 
   @override
   Widget build(BuildContext context) {