feat(app): токены темы — тёмный сине-индиго фон, единый синий акцент

ColorScheme в AppTheme.light()/.dark() строится из одного seed вместо
двух параллельных наборов чисел для тёмной и светлой темы — акцент
сведён к ChartColors.slot1Blue, чтобы фирменный цвет интерфейса и
основной цвет графиков совпадали. cardTheme/navigationRailTheme/
navigationBarTheme заданы централизованно и каскадом меняют вид
существующих карточек и навигации на всех экранах.

app/lib/app.dart

@@ -2,6 +2,7 @@ import 'package:flutter/material.dart';
 import 'package:flutter_localizations/flutter_localizations.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 
+import 'core/theme/app_theme.dart';
 import 'core/theme/theme_controller.dart';
 import 'router.dart';
 
@@ -16,12 +17,8 @@ class FinTrackerApp extends ConsumerWidget {
       title: 'fin-tracker',
       routerConfig: router,
       themeMode: themeMode,
-      theme: ThemeData(colorSchemeSeed: const Color(0xFF2E6F5E), useMaterial3: true),
+      theme: AppTheme.light(),
-      darkTheme: ThemeData(
+      darkTheme: AppTheme.dark(),
-        colorSchemeSeed: const Color(0xFF2E6F5E),
-        brightness: Brightness.dark,
-        useMaterial3: true,
-      ),
       locale: const Locale('ru'),
       supportedLocales: const [Locale('ru'), Locale('en')],
       localizationsDelegates: GlobalMaterialLocalizations.delegates,

app/lib/core/theme/app_theme.dart

@@ -0,0 +1,75 @@
+import 'package:flutter/material.dart';
+
+/// Design tokens for fin-tracker's visual language (`docs/ai/design-system.md`): dark
+/// surfaces tinted blue-indigo rather than neutral grey, flat bordered cards instead of
+/// Material's default elevation shadow, and one accent seed shared with
+/// `ChartColors.slot1Blue` so UI chrome and the primary data series read as the same color.
+///
+/// Both brightnesses share this seed and this shape language — only the surface tones
+/// differ — so the dark theme is not a special case bolted onto a default light Material
+/// theme, and switching `themeModeProvider` never changes which widgets exist.
+class AppTheme {
+  const AppTheme._();
+
+  /// Matches `ChartColors.slot1Blue`.
+  static const seed = Color(0xFF2A78D6);
+
+  static ThemeData light() => _build(Brightness.light);
+  static ThemeData dark() => _build(Brightness.dark);
+
+  static ThemeData _build(Brightness brightness) {
+    final scheme = ColorScheme.fromSeed(seedColor: seed, brightness: brightness);
+    final outline = scheme.outlineVariant.withValues(alpha: brightness == Brightness.dark ? 0.4 : 0.7);
+
+    return ThemeData(
+      useMaterial3: true,
+      brightness: brightness,
+      colorScheme: scheme,
+      scaffoldBackgroundColor: scheme.surface,
+      appBarTheme: AppBarTheme(
+        backgroundColor: scheme.surface,
+        surfaceTintColor: Colors.transparent,
+        elevation: 0,
+        scrolledUnderElevation: 0,
+      ),
+      cardTheme: CardThemeData(
+        elevation: 0,
+        color: scheme.surfaceContainer,
+        surfaceTintColor: Colors.transparent,
+        clipBehavior: Clip.antiAlias,
+        margin: EdgeInsets.zero,
+        shape: RoundedRectangleBorder(
+          borderRadius: BorderRadius.circular(16),
+          side: BorderSide(color: outline),
+        ),
+      ),
+      dividerTheme: DividerThemeData(color: outline, space: 1, thickness: 1),
+      navigationRailTheme: NavigationRailThemeData(
+        backgroundColor: scheme.surfaceContainerLow,
+        indicatorColor: scheme.primaryContainer,
+        useIndicator: true,
+        selectedIconTheme: IconThemeData(color: scheme.primary),
+        unselectedIconTheme: IconThemeData(color: scheme.onSurfaceVariant),
+        selectedLabelTextStyle: TextStyle(color: scheme.primary, fontWeight: FontWeight.w600),
+        unselectedLabelTextStyle: TextStyle(color: scheme.onSurfaceVariant),
+      ),
+      navigationBarTheme: NavigationBarThemeData(
+        backgroundColor: scheme.surfaceContainerLow,
+        indicatorColor: scheme.primaryContainer,
+        elevation: 0,
+        iconTheme: WidgetStateProperty.resolveWith(
+          (states) => IconThemeData(
+            color: states.contains(WidgetState.selected) ? scheme.primary : scheme.onSurfaceVariant,
+          ),
+        ),
+        labelTextStyle: WidgetStateProperty.resolveWith(
+          (states) => TextStyle(
+            fontSize: 12,
+            fontWeight: states.contains(WidgetState.selected) ? FontWeight.w600 : FontWeight.w400,
+            color: states.contains(WidgetState.selected) ? scheme.primary : scheme.onSurfaceVariant,
+          ),
+        ),
+      ),
+    );
+  }
+}