Improve decimal handling

### lib/models/app_wallet.dart
@@ -39,6 +39,10 @@ abstract interface class AppWallet implements Listenable {
 
   // Balances + history
   double? get unlockedBalance;
+
+  /// The same balance, exactly. [unlockedBalance] is a double for display and
+  /// fiat maths; anything that becomes a spend amount must start here.
+  BigInt? get unlockedBalanceBaseUnits;
   double? get totalBalance;
   List<TxDetails> get txHistory;
   bool? get serverSupportsSubaddresses;
@@ -110,18 +114,14 @@ abstract interface class AppWallet implements Listenable {
 
   /// Estimated network fee in base units (piconero), or null when the backend
   /// can't estimate it (typically insufficient balance for that priority).
-  Future<int?> estimateFee(
-    String destinationAddress,
-    double amount, {
-    int priority,
-    String? amountText,
-  });
+  /// [amount] is exact decimal text, never a double. A spend amount that has
+  /// been through a double is no longer the amount the user asked for.
+  Future<int?> estimateFee(String destinationAddress, String amount, {int priority});
   Future<AppPendingTx> createTx(
     String destinationAddress,
-    double amount,
+    String amount,
     bool isSweepAll, {
     int priority,
-    String? amountText,
   });
   Future<void> commitTx(AppPendingTx tx, String destinationAddress);
   Future<ResolvedOpenAlias?> resolveOpenAlias(String alias);

### lib/models/monero_wallet_adapter.dart
@@ -73,6 +73,8 @@ class MoneroWalletAdapter extends ChangeNotifier implements AppWallet {
   @override
   double? get unlockedBalance => _wallet.unlockedBalance;
   @override
+  BigInt? get unlockedBalanceBaseUnits => _wallet.unlockedBalanceBaseUnits;
+  @override
   double? get totalBalance => _wallet.totalBalance;
   @override
   bool? get serverSupportsSubaddresses => _wallet.serverSupportsSubaddresses;
@@ -207,15 +209,10 @@ class MoneroWalletAdapter extends ChangeNotifier implements AppWallet {
   bool isAddressValid(String address) => _wallet.isAddressValid(address);
 
   @override
-  Future<int?> estimateFee(
-    String destinationAddress,
-    double amount, {
-    int priority = 0,
-    String? amountText,
-  }) async {
+  Future<int?> estimateFee(String destinationAddress, String amount, {int priority = 0}) async {
     final fee = await _wallet.estimateFee(
       destinationAddress,
-      _toBaseUnits(amount, amountText),
+      domain.decimalToBaseUnits(amount, _moneroDecimals),
       priority: priority,
     );
     return fee?.toInt();
@@ -224,14 +221,13 @@ class MoneroWalletAdapter extends ChangeNotifier implements AppWallet {
   @override
   Future<AppPendingTx> createTx(
     String destinationAddress,
-    double amount,
+    String amount,
     bool isSweepAll, {
     int priority = 0,
-    String? amountText,
   }) async {
     final tx = await _wallet.createTx(
       destinationAddress,
-      _toBaseUnits(amount, amountText),
+      domain.decimalToBaseUnits(amount, _moneroDecimals),
       isSweepAll,
       priority: priority,
     );
@@ -258,12 +254,6 @@ class MoneroWalletAdapter extends ChangeNotifier implements AppWallet {
       return null;
     }
   }
-
-  // Prefer the exact decimal string; a double loses precision (D4).
-  static BigInt _toBaseUnits(double amount, String? amountText) => domain.decimalToBaseUnits(
-    amountText ?? amount.toStringAsFixed(_moneroDecimals),
-    _moneroDecimals,
-  );
 }
 
 /// Wraps a wallet-core [domain.PendingTransaction] as the neutral [AppPendingTx].

### lib/screens/send.dart
@@ -2,6 +2,7 @@ import 'package:flutter/material.dart';
 import 'package:flutter/services.dart';
 import 'package:flutter_svg/flutter_svg.dart';
 import 'package:provider/provider.dart';
+import 'package:wallet_domain/wallet_domain.dart' show baseUnitsToDecimalString, decimalToBaseUnits;
 
 import 'package:skylight_wallet/consts.dart' as consts;
 import 'package:skylight_wallet/l10n/app_localizations.dart';
@@ -16,7 +17,9 @@ import 'package:skylight_wallet/widgets/ui/ui.dart';
 
 class SendScreenArgs {
   String destinationAddress;
-  double? amount;
+
+  /// Exact decimal text, never a double: this is a spend amount.
+  String? amount;
 
   /// Set when the address came from a contact (e.g. Send in the address book),
   /// so Send opens showing the contact card rather than a bare address.
@@ -109,7 +112,7 @@ class _SendScreenState extends State<SendScreen> {
 
     if (args != null) {
       _destinationAddressController.text = args.destinationAddress;
-      _amountController.text = args.amount != null ? args.amount.toString() : '';
+      _amountController.text = args.amount ?? '';
       // Same field the in-send picker sets, so the contact card renders here too.
       _selectedContact = args.contact;
     }
@@ -132,7 +135,7 @@ class _SendScreenState extends State<SendScreen> {
     if (result == null || result is! String) return;
 
     String address = '';
-    double? amount;
+    String? amount;
     final uri = Uri.tryParse(result);
 
     if (uri != null && uri.scheme == 'monero') {
@@ -145,9 +148,7 @@ class _SendScreenState extends State<SendScreen> {
 
       address = uri.path;
 
-      if (uri.queryParameters.containsKey('tx_amount')) {
-        amount = double.tryParse(uri.queryParameters['tx_amount']!);
-      }
+      amount = uri.queryParameters['tx_amount'];
     } else if (wallet.isAddressValid(result)) {
       address = result;
     } else {
@@ -159,7 +160,24 @@ class _SendScreenState extends State<SendScreen> {
 
     _destinationAddressController.text = address;
     if (amount != null) {
-      _amountController.text = amount.toString();
+      _amountController.text = _asExactAmount(amount);
+    }
+  }
+
+  /// A scanned amount as Monero can actually express it.
+  ///
+  /// Text the whole way. Parsing to a double and back is what used to change
+  /// the value; base units are exact, so this round trip can only drop digits
+  /// finer than one piconero — and it shows the user the amount that will
+  /// really be spent instead of one that gets truncated later.
+  ///
+  /// Unparseable input goes in the field verbatim, so the form's own validation
+  /// rejects it and the user can see what was scanned.
+  String _asExactAmount(String raw) {
+    try {
+      return baseUnitsToDecimalString(decimalToBaseUnits(raw, _xmrDecimals), _xmrDecimals);
+    } on FormatException {
+      return raw;
     }
   }
 
@@ -332,14 +350,13 @@ class _SendScreenState extends State<SendScreen> {
 
     final destinationAddress = await _resolveDestinationAddress();
     final amountText = _amountController.text;
-    final amount = double.parse(amountText);
 
     try {
       // Estimate the fee per priority natively (no full tx build).
       final fees = await Future.wait([
-        wallet.estimateFee(destinationAddress, amount, priority: 1, amountText: amountText),
-        wallet.estimateFee(destinationAddress, amount, priority: 2, amountText: amountText),
-        wallet.estimateFee(destinationAddress, amount, priority: 3, amountText: amountText),
+        wallet.estimateFee(destinationAddress, amountText, priority: 1),
+        wallet.estimateFee(destinationAddress, amountText, priority: 2),
+        wallet.estimateFee(destinationAddress, amountText, priority: 3),
       ]);
 
       // Only update state if this is still the latest request
@@ -398,7 +415,6 @@ class _SendScreenState extends State<SendScreen> {
     }
 
     final destinationAddressUnresolved = _destinationAddressController.text;
-    final amount = double.parse(_amountController.text);
     String destinationAddress = '';
     String? destinationOpenAlias;
     String? destinationOpenAliasName;
@@ -428,10 +444,9 @@ class _SendScreenState extends State<SendScreen> {
       // screen are estimates, not tx objects, so always construct here).
       final tx = await wallet.createTx(
         destinationAddress,
-        amount,
+        _amountController.text,
         _isSweepAll,
         priority: _selectedPriority + 1,
-        amountText: _amountController.text,
       );
 
       setState(() {
@@ -448,7 +463,10 @@ class _SendScreenState extends State<SendScreen> {
       }
     } catch (error) {
       if (error.toString().contains('Unlocked funds too low')) {
-        if (wallet.unlockedBalance! > amount) {
+        // Display-only: picks which of two error messages to show, so the
+        // imprecision of a double cannot reach an amount anyone spends.
+        final approxAmount = double.tryParse(_amountController.text) ?? 0;
+        if (wallet.unlockedBalance! > approxAmount) {
           setState(() {
             _amountError = i18n.sendInsufficientBalanceToCoverFeeError;
           });
@@ -550,7 +568,12 @@ class _SendScreenState extends State<SendScreen> {
 
   void _setBalanceAsSendAmount() {
     final wallet = appWalletOf(context);
-    _amountController.text = _plainAmount(wallet.unlockedBalance ?? 0);
+    // From base units, not from the display double: Max fills a field that is
+    // about to be spent, and a balance over ~9007 XMR does not survive a double
+    // intact. Sweep-all normally makes the amount moot, but editing the field
+    // clears that flag and the number becomes the real amount.
+    final units = wallet.unlockedBalanceBaseUnits;
+    _amountController.text = units == null ? '' : baseUnitsToDecimalString(units, _xmrDecimals);
 
     setState(() {
       _isSweepAll = true;
@@ -701,20 +724,6 @@ String _amountText(double amount) => amount.toStringAsFixed(5);
 /// this file does not reach into the adapter's privates.
 const _xmrDecimals = 12;
 
-/// [amount] as a plain decimal, never exponential.
-///
-/// `double.toString()` switches to exponential notation below 1e-6 -- a
-/// 500000-piconero balance renders as "5e-7". `decimalToBaseUnits` splits on
-/// '.' and hands the rest to `BigInt.parse`, which throws on an exponent, so
-/// the fee estimate fails and the field cannot be sent. Only the Max button
-/// fills this field from a raw double, which is why it only broke there.
-String _plainAmount(double amount) {
-  final fixed = amount.toStringAsFixed(_xmrDecimals);
-  if (!fixed.contains('.')) return fixed;
-  final trimmed = fixed.replaceFirst(RegExp(r'0+$'), '');
-  return trimmed.endsWith('.') ? trimmed.substring(0, trimmed.length - 1) : trimmed;
-}
-
 /// `abcd…wxyz`: keeps [head] leading and [tail] trailing chars of a long
 /// address, eliding the middle. Returns the string unchanged when short.
 String _shortenMiddle(String value, {required int head, required int tail}) {