fix(payjoin): format BIP21 amount as plain decimal

double.toString() emits scientific notation below 1e-6 (e.g. amount=1e-8
for 1 sat), which is not a valid BIP21 amount. Add formatBtcAmount (8
decimals, trailing zeros trimmed) and apply it to the payjoin, bitcoin
and liquid receive paths. Add tests covering sub-100-sat amounts.

Author: ethicnology

lib/features/receive/presentation/bloc/receive_state.dart

@@ -111,7 +111,7 @@ abstract class ReceiveState with _$ReceiveState {
           scheme: 'bitcoin',
           path: bitcoinAddress!.address,
           queryParameters: {
-            if (confirmedAmountBtc > 0) 'amount': confirmedAmountBtc.toString(),
+            if (confirmedAmountBtc > 0) 'amount': formatBtcAmount(confirmedAmountBtc),
             if (note.isNotEmpty) 'message': note,
           },
         );
@@ -128,7 +128,7 @@ abstract class ReceiveState with _$ReceiveState {
           scheme: 'liquidnetwork',
           path: liquidAddress!.address,
           queryParameters: {
-            if (confirmedAmountBtc > 0) 'amount': confirmedAmountBtc.toString(),
+            if (confirmedAmountBtc > 0) 'amount': formatBtcAmount(confirmedAmountBtc),
             if (note.isNotEmpty) 'message': note,
             'assetid':
                 wallet != null && wallet!.network == Network.liquidMainnet
@@ -163,13 +163,26 @@ abstract class ReceiveState with _$ReceiveState {
     final base = pjUri.substring(0, q); // bitcoin:<address>
     final pdkQuery = pjUri.substring(q + 1); // [pjos=0&]pj=...
     final extras = <String>[
-      if (amountBtc > 0) 'amount=$amountBtc',
+      if (amountBtc > 0) 'amount=${formatBtcAmount(amountBtc)}',
       if (note.isNotEmpty) 'message=${Uri.encodeQueryComponent(note)}',
       if (!pdkQuery.contains('pjos=')) 'pjos=0',
     ];
     return extras.isEmpty ? pjUri : '$base?${extras.join('&')}&$pdkQuery';
   }
 
+  /// Formats a BTC amount as a plain decimal for a BIP21 `amount` parameter.
+  ///
+  /// `double.toString()` switches to scientific notation below 1e-6 (e.g.
+  /// `1e-8` for 1 sat), which is not a valid BIP21 amount. Format with 8
+  /// decimals (sat precision) and trim trailing zeros.
+  static String formatBtcAmount(double amountBtc) {
+    var s = amountBtc.toStringAsFixed(8);
+    if (s.contains('.')) {
+      s = s.replaceAll(RegExp(r'0+$'), '').replaceAll(RegExp(r'\.$'), '');
+    }
+    return s;
+  }
+
   String get addressOrInvoiceOnly {
     switch (type) {
       case ReceiveType.bitcoin:

test/features/receive/receive_payjoin_uri_test.dart

@@ -75,6 +75,47 @@ void main() {
     });
   });
 
+  group('formatBtcAmount', () {
+    test('never uses scientific notation for small amounts', () {
+      // 1 sat and 10 sats would render as 1e-8 / 1e-7 via double.toString().
+      expect(ReceiveState.formatBtcAmount(0.00000001), equals('0.00000001'));
+      expect(ReceiveState.formatBtcAmount(0.0000001), equals('0.0000001'));
+      expect(ReceiveState.formatBtcAmount(0.000001), equals('0.000001'));
+    });
+
+    test('trims trailing zeros', () {
+      expect(ReceiveState.formatBtcAmount(0.001), equals('0.001'));
+      expect(ReceiveState.formatBtcAmount(1.0), equals('1'));
+      expect(ReceiveState.formatBtcAmount(0.00666666), equals('0.00666666'));
+    });
+  });
+
+  group('regression: the previous dart:core Uri reconstruction', () {
+    // Reproduces the old implementation to document precisely why it was
+    // replaced: round-tripping the PDK pjUri through dart:core Uri
+    // percent-encoded the :// and dropped pjos.
+    test('over-encoded :// and dropped pjos', () {
+      final pjUri = Uri.parse(pdkPjUri);
+      var bip21Uri = Uri(
+        scheme: 'bitcoin',
+        path: 'tb1q6q6de88mj8qkg0q5lupmpfexwnqjsr4d2gvx2p',
+        queryParameters: {'amount': '0.001'},
+      );
+      bip21Uri = bip21Uri.replace(
+        queryParameters: {
+          ...bip21Uri.queryParameters,
+          if (pjUri.queryParameters['pjos'] != null)
+            'pjos': pjUri.queryParameters['pjos']!,
+          'pj': pjUri.queryParameters['pj']!,
+        },
+      );
+      final old = bip21Uri.toString();
+      expect(old.contains('%3A%2F%2F'), isTrue, reason: ':// over-encoded');
+      expect(old.contains('HTTPS://'), isFalse);
+      expect(old.contains('pjos'), isFalse, reason: 'pjos dropped');
+    });
+  });
+
   group('generated URI is consumable by the bip21_uri decoder', () {
     test('the pj endpoint survives a decode round-trip', () {
       final generated = ReceiveState.buildPayjoinPaymentRequest(