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/theme/chart_colors.dart

@@ -0,0 +1,21 @@
+import 'package:flutter/material.dart';
+
+/// Fixed categorical order from the design system's palette (dataviz skill):
+/// slot 1 blue, slot 2 orange, slot 3 aqua, slot 4 yellow, slot 5 magenta.
+/// Assigned to entities in this fixed order — never cycled, never by rank.
+class ChartColors {
+  const ChartColors._();
+
+  static const slot1Blue = Color(0xFF2A78D6);
+  static const slot2Orange = Color(0xFFEB6834);
+  static const slot3Aqua = Color(0xFF1BAF7A);
+  static const slot4Yellow = Color(0xFFEDA100);
+  static const slot5Magenta = Color(0xFFE87BA4);
+
+  /// This app's fixed assignment for cashflow charts.
+  static const income = slot1Blue;
+  static const expense = slot2Orange;
+  static const baseline = slot3Aqua;
+  static const oneOff = slot4Yellow;
+  static const savingsTransfer = slot5Magenta;
+}

app/lib/core/theme/theme_controller.dart

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+import 'package:flutter/material.dart';
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+import 'package:shared_preferences/shared_preferences.dart';
+
+const _prefsKey = 'theme_mode';
+
+final themeModeProvider = NotifierProvider<ThemeModeController, ThemeMode>(ThemeModeController.new);
+
+/// Persists the chosen [ThemeMode] to this device via `shared_preferences`.
+class ThemeModeController extends Notifier<ThemeMode> {
+  @override
+  ThemeMode build() {
+    Future.microtask(_restore);
+    return ThemeMode.system;
+  }
+
+  Future<void> _restore() async {
+    final prefs = await SharedPreferences.getInstance();
+    final saved = prefs.getString(_prefsKey);
+    state = ThemeMode.values.asNameMap()[saved] ?? ThemeMode.system;
+  }
+
+  Future<void> setMode(ThemeMode mode) async {
+    state = mode;
+    final prefs = await SharedPreferences.getInstance();
+    await prefs.setString(_prefsKey, mode.name);
+  }
+}