refactor(legacy): migrate theming to Material 3 + SfColors extension

Replace the ThemePort/ThemeCode abstraction (GetIt-registered adapter)
with a Riverpod-driven Material 3 ColorScheme, an SfColors ThemeExtension
for brand tokens, and a user-facing appearance selector for light/dark/
system modes. Persisted via SharedPreferences, reacts to system
brightness changes. Payments mode keeps the existing ThemePort API.

Highlights
- New legacy_theme package: LegacyAppTheme (light/dark), LegacyColorSchemes,
  SfColors ThemeExtension, LegacyThemePreferences, LegacyThemeNotifier,
  LegacyThemeSelector. Timeframe-based variants scaffolded but disabled.
- New /legacy/dashboard/control_panel/settings/appearance route + screen.
- MaterialApp.router picks the legacy theme only when isLegacyMode.
- ~90 ThemeCode.* usages migrated to colorScheme.* / context.sfColors.*.
- 25 widgets dropped the 'ThemePort theme' constructor param.
- ~145 hardcoded colors migrated (exact hex 1:1, grey.shade tiers,
  destructive red -> colorScheme.error, background whites -> surface).
  Content-over-color whites, transparents, and brand semantic reds/
  oranges/greens intentionally preserved.
- sf_localizations updated with appearance / appearanceDescription keys
  in all six locales.

apps/mobile_app/lib/core/init_app.dart

@@ -5,6 +5,7 @@ import 'package:flutter/services.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 import 'package:intl/date_symbol_data_local.dart';
 import 'package:design_system/design_system.dart';
+import 'package:legacy_theme/legacy_theme.dart';
 import 'package:sca_treezor/sca_treezor.dart';
 import 'package:sf_app_platform/config/env/environment_enum.dart';
 import 'package:sf_app_platform/config/env/save_family_env_config.dart';
@@ -16,12 +17,15 @@ import 'package:sf_app_platform/save_family_app.dart';
 import 'package:navigation/navigation.dart';
 import 'package:sf_infrastructure/sf_infrastructure.dart';
 import 'package:sf_tracking/sf_tracking.dart';
+import 'package:shared_preferences/shared_preferences.dart';
 
 Future<void> initApp(EnvironmentEnum env) async {
   WidgetsFlutterBinding.ensureInitialized();
   await SystemChrome.setPreferredOrientations([DeviceOrientation.portraitUp]);
   await initializeDateFormatting();
 
+  final sharedPreferences = await SharedPreferences.getInstance();
+
   navigationModule();
   scaTreezorModule();
   themePackages();
@@ -52,5 +56,12 @@ Future<void> initApp(EnvironmentEnum env) async {
     },
   );
 
-  runApp(const ProviderScope(child: SaveFamilyApp()));
+  runApp(
+    ProviderScope(
+      overrides: [
+        sharedPreferencesProvider.overrideWithValue(sharedPreferences),
+      ],
+      child: const SaveFamilyApp(),
+    ),
+  );
 }

apps/mobile_app/lib/navigation/app_router.dart

@@ -250,6 +250,11 @@ void configureAppRouter() {
                 name: 'settings',
                 pageBuilder: SettingsBuilder().buildPage,
                 routes: [
+                  GoRoute(
+                    path: 'appearance',
+                    name: 'appearance',
+                    pageBuilder: AppearanceBuilder().buildPage,
+                  ),
                   GoRoute(
                     path: 'alarm',
                     name: 'alarm',

apps/mobile_app/lib/save_family_app.dart

@@ -3,6 +3,7 @@ import 'package:firebase_messaging/firebase_messaging.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter_localizations/flutter_localizations.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
+import 'package:legacy_theme/legacy_theme.dart';
 import 'package:sf_app_platform/core/app_version_check/app_update_gate.dart';
 import 'package:sf_app_platform/core/app_version_check/app_version_check.dart';
 import 'package:sf_app_platform/core/config/app_mode.dart';
@@ -130,13 +131,32 @@ class SaveFamilyAppState extends ConsumerState<SaveFamilyApp>
     SizeUtils.init(context: context);
     ref.watch(pushTokenRefreshListenerProvider);
 
+    // Theme wiring:
+    // - Legacy mode: new `legacy_theme` package (Material 3 + light/dark/system).
+    // - Payment mode: unchanged behaviour (seed-based ColorScheme, light only).
+    final ThemeData lightTheme;
+    final ThemeData? darkTheme;
+    final ThemeMode themeMode;
+    if (isLegacyMode) {
+      final legacyThemeState = ref.watch(legacyThemeNotifierProvider);
+      lightTheme = LegacyAppTheme.light;
+      darkTheme = LegacyAppTheme.dark;
+      themeMode = legacyThemeState.themeMode;
+    } else {
+      lightTheme = ThemeData(
+        fontFamily: AppFonts.stolzl,
+        colorScheme: ColorScheme.fromSeed(seedColor: const Color(0xFF329E95)),
+      );
+      darkTheme = null;
+      themeMode = ThemeMode.light;
+    }
+
     return AppUpdateGate(
       child: MaterialApp.router(
         title: 'SaveFamily',
-        theme: ThemeData(
-          fontFamily: AppFonts.stolzl,
-          colorScheme: ColorScheme.fromSeed(seedColor: const Color(0xFF329E95)),
-        ),
+        theme: lightTheme,
+        darkTheme: darkTheme,
+        themeMode: themeMode,
         routerConfig: appRouter,
         debugShowCheckedModeBanner: false,
         localizationsDelegates: [