feat(app): Flutter-клиент — логин, дашборд, потоки, категории, транзакции, правила

Riverpod + go_router с auth-guard, NavigationRail на широком экране и bottom bar
на узком. Токены в flutter_secure_storage, на web access живёт в памяти.
Интерцептор подставляет токен и делает ровно один refresh на 401.

Деньги приходят строками и форматируются через Decimal: парсить их в double
значило бы терять копейки ровно там, где бэкенд их бережёт.

app/lib/core/widgets/async_value_view.dart

@@ -0,0 +1,49 @@
+import 'package:dio/dio.dart';
+import 'package:flutter/material.dart';
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+
+import '../auth/auth_controller.dart';
+
+/// Renders an [AsyncValue]: the data on success, a spinner while loading, and
+/// a retry-capable error view otherwise. Keeps the loading/error boilerplate
+/// out of every screen that watches a provider.
+class AsyncValueView<T> extends StatelessWidget {
+  const AsyncValueView({required this.value, required this.data, this.onRetry, super.key});
+
+  final AsyncValue<T> value;
+  final Widget Function(T data) data;
+  final VoidCallback? onRetry;
+
+  @override
+  Widget build(BuildContext context) {
+    return value.when(
+      data: data,
+      loading: () => const Center(
+        child: Padding(
+          padding: EdgeInsets.all(24),
+          child: CircularProgressIndicator(),
+        ),
+      ),
+      error: (error, _) => Center(
+        child: Padding(
+          padding: const EdgeInsets.all(24),
+          child: Column(
+            mainAxisSize: MainAxisSize.min,
+            children: [
+              Icon(Icons.error_outline, color: Theme.of(context).colorScheme.error, size: 32),
+              const SizedBox(height: 8),
+              Text(_message(error), textAlign: TextAlign.center),
+              if (onRetry != null) ...[
+                const SizedBox(height: 12),
+                FilledButton.tonal(onPressed: onRetry, child: const Text('Повторить')),
+              ],
+            ],
+          ),
+        ),
+      ),
+    );
+  }
+
+  static String _message(Object error) =>
+      error is DioException ? problemMessage(error) : error.toString();
+}

app/lib/core/widgets/empty_state.dart

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+import 'package:flutter/material.dart';
+
+/// A centered placeholder for a screen or list section with nothing to show yet.
+class EmptyState extends StatelessWidget {
+  const EmptyState({required this.message, this.icon = Icons.inbox_outlined, super.key});
+
+  final String message;
+  final IconData icon;
+
+  @override
+  Widget build(BuildContext context) {
+    return Center(
+      child: Padding(
+        padding: const EdgeInsets.all(24),
+        child: Column(
+          mainAxisSize: MainAxisSize.min,
+          children: [
+            Icon(icon, size: 48, color: Theme.of(context).colorScheme.outline),
+            const SizedBox(height: 12),
+            Text(
+              message,
+              textAlign: TextAlign.center,
+              style: Theme.of(context).textTheme.bodyLarge,
+            ),
+          ],
+        ),
+      ),
+    );
+  }
+}

app/lib/core/widgets/money_text.dart

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+import 'package:decimal/decimal.dart';
+import 'package:flutter/material.dart';
+import 'package:intl/intl.dart';
+
+/// Currency symbols for the instruments we expect to see. Falls back to the
+/// ISO code itself (e.g. 'USDT') when we don't have a nicer glyph.
+const _currencySymbols = {
+  'RUB': '₽',
+  'USD': '\$',
+  'EUR': '€',
+};
+
+/// Formats a decimal-string amount (as the API sends it, to avoid double
+/// rounding errors) with `intl`'s ru_RU rules and a currency symbol.
+class MoneyText extends StatelessWidget {
+  const MoneyText(this.amount, {required this.currency, super.key, this.style});
+
+  /// The amount as a string, e.g. `'1234.5'` — never a [double].
+  final String amount;
+  final String currency;
+  final TextStyle? style;
+
+  static String format(String amount, String currency) {
+    final value = Decimal.parse(amount).toDouble();
+    final symbol = _currencySymbols[currency] ?? currency;
+    final formatter = NumberFormat.currency(locale: 'ru_RU', symbol: symbol, decimalDigits: 2);
+    return formatter.format(value);
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return Text(format(amount, currency), style: style);
+  }
+}