Refactor theme management: Replace AppColors with AppPalette

- Removed AppColors class and migrated all references to AppPalette.
- Updated VaultAccessScreen, MenuDrawer, NoteCard, SearchAppBar, and other widgets to use AppPalette for color management.
- Introduced AppPalette to handle light and dark themes with appropriate color schemes.
- Adjusted theme application in AppTheme to utilize AppPalette extensions.
- Updated tests to reflect changes in theme structure and color references.

lib/app.dart

@@ -14,7 +14,7 @@ import 'package:notas/screens/biometric_gate_screen.dart';
 import 'package:notas/screens/home_screen.dart';
 import 'package:notas/screens/settings_screen.dart';
 import 'package:notas/screens/vault_access_screen.dart';
-import 'package:notas/theme/app_colors.dart';
+import 'package:notas/theme/app_palette.dart';
 import 'package:notas/theme/app_theme.dart';
 import 'package:notas/widgets/sync_status.dart';
 import 'package:path/path.dart' as p;
@@ -44,6 +44,7 @@ class _NotesAppState extends State<NotesApp>
   static const Duration _syncInterval = Duration(minutes: 5);
   static const Duration _windowSizeSaveDelay = Duration(milliseconds: 350);
   static const String _themeSeedColorKey = 'theme_seed_color_v1';
+  static const String _themeModeKey = 'theme_mode_v1';
 
   final LocalVaultService _vaultService = LocalVaultService.instance;
   final GlobalKey<ScaffoldMessengerState> _scaffoldMessengerKey =
@@ -69,7 +70,30 @@ class _NotesAppState extends State<NotesApp>
   String? _syncErrorMessage;
   int _syncOperationId = 0;
   int _homeRefreshToken = 0;
-  Color _themeSeedColor = AppColors.defaultThemeSeedColor;
+  Color _themeSeedColor = Colors.amber;
+  ThemeMode _themeMode = ThemeMode.system;
+
+  // Cached ThemeData for light and dark variants.
+  ThemeData? _lightTheme;
+  ThemeData? _darkTheme;
+
+  Brightness _effectiveBrightness() {
+    switch (_themeMode) {
+      case ThemeMode.dark:
+        return Brightness.dark;
+      case ThemeMode.light:
+        return Brightness.light;
+      case ThemeMode.system:
+        return WidgetsBinding.instance.platformDispatcher.platformBrightness;
+    }
+  }
+
+  AppPalette _activePalette() {
+    return AppPalette.fromBrightness(
+      _effectiveBrightness(),
+      seedColor: _themeSeedColor,
+    );
+  }
 
   @override
   void initState() {
@@ -80,6 +104,7 @@ class _NotesAppState extends State<NotesApp>
       windowManager.setPreventClose(true);
     }
     _loadThemeSeedColor();
+    _loadThemeMode();
     _bootstrapVault();
   }
 
@@ -106,7 +131,7 @@ class _NotesAppState extends State<NotesApp>
 
     setState(() {
       _themeSeedColor = Color(storedColorValue);
-      _themeData = AppTheme.theme(seedColor: _themeSeedColor);
+      _updateThemeData();
     });
   }
 
@@ -120,15 +145,54 @@ class _NotesAppState extends State<NotesApp>
 
     setState(() {
       _themeSeedColor = color;
-      _themeData = AppTheme.theme(seedColor: _themeSeedColor);
+      _updateThemeData();
     });
   }
 
-  // Cached ThemeData to avoid recomputing on every build.
-  ThemeData? _themeData;
+  Future<void> _loadThemeMode() async {
+    final SharedPreferences prefs = await SharedPreferences.getInstance();
+    final int? stored = prefs.getInt(_themeModeKey);
+    if (!mounted) return;
 
-  ThemeData get _theme =>
-      _themeData ??= AppTheme.theme(seedColor: _themeSeedColor);
+    setState(() {
+      if (stored == null) {
+        _themeMode = ThemeMode.system;
+      } else if (stored == 1) {
+        _themeMode = ThemeMode.light;
+      } else if (stored == 2) {
+        _themeMode = ThemeMode.dark;
+      } else {
+        _themeMode = ThemeMode.system;
+      }
+      _updateThemeData();
+    });
+  }
+
+  Future<void> _setThemeMode(ThemeMode mode) async {
+    final SharedPreferences prefs = await SharedPreferences.getInstance();
+    final int stored = mode == ThemeMode.system
+        ? 0
+        : (mode == ThemeMode.light ? 1 : 2);
+    await prefs.setInt(_themeModeKey, stored);
+
+    if (!mounted) return;
+
+    setState(() {
+      _themeMode = mode;
+    });
+  }
+
+  void _updateThemeData() {
+    _lightTheme = AppTheme.theme(
+      seedColor: _themeSeedColor,
+      brightness: Brightness.light,
+    );
+    _darkTheme = AppTheme.theme(
+      seedColor: _themeSeedColor,
+      brightness: Brightness.dark,
+    );
+    // Updated light/dark themes regenerated
+  }
 
   @override
   void didChangeAppLifecycleState(AppLifecycleState state) {
@@ -429,27 +493,30 @@ class _NotesAppState extends State<NotesApp>
 
             final bool? retry = await showDialog<bool>(
               context: dialogCtx,
-              builder: (BuildContext context) => AlertDialog(
-              backgroundColor: AppColors.cardBackground,
-              shape: RoundedRectangleBorder(
-                borderRadius: BorderRadius.circular(12),
-                side: BorderSide(color: AppColors.border),
-              ),
-                title: const Text('No se pudo activar la biometría'),
-                content: const Text(
-                  'No se pudo activar la biometría. ¿Quieres intentarlo de nuevo o entrar sin huella?',
-                ),
-                actions: [
-                  TextButton(
-                    onPressed: () => navigator.pop(false),
-                    child: const Text('Entrar sin huella'),
+              builder: (BuildContext context) {
+                final AppPalette palette = _activePalette();
+                return AlertDialog(
+                  backgroundColor: palette.cardBackground,
+                  shape: RoundedRectangleBorder(
+                    borderRadius: BorderRadius.circular(12),
+                    side: BorderSide(color: palette.border),
                   ),
-                  FilledButton(
-                    onPressed: () => navigator.pop(true),
-                    child: const Text('Reintentar'),
+                  title: const Text('No se pudo activar la biometría'),
+                  content: const Text(
+                    'No se pudo activar la biometría. ¿Quieres intentarlo de nuevo o entrar sin huella?',
                   ),
-                ],
-              ),
+                  actions: [
+                    TextButton(
+                      onPressed: () => navigator.pop(false),
+                      child: const Text('Entrar sin huella'),
+                    ),
+                    FilledButton(
+                      onPressed: () => navigator.pop(true),
+                      child: const Text('Reintentar'),
+                    ),
+                  ],
+                );
+              },
             );
 
             if (retry != true) {
@@ -825,6 +892,7 @@ class _NotesAppState extends State<NotesApp>
   }
 
   Widget _buildMainShell(NoteRepository repository) {
+    final AppPalette palette = _activePalette();
     final Widget activeScreen = _currentSection == _AppSection.home
         ? HomeScreen(
             key: const ValueKey<String>('home-screen'),
@@ -845,6 +913,8 @@ class _NotesAppState extends State<NotesApp>
             onForceSync: () => _performSync(forceFull: true),
             currentSeedColor: _themeSeedColor,
             onThemeColorSelected: _setThemeSeedColor,
+            currentThemeMode: _themeMode,
+            onThemeModeSelected: _setThemeMode,
           );
 
     return Shortcuts(
@@ -864,9 +934,7 @@ class _NotesAppState extends State<NotesApp>
           autofocus: true,
           child: Scaffold(
             body: Container(
-              decoration: const BoxDecoration(
-                gradient: AppColors.backdropGradient,
-              ),
+              decoration: BoxDecoration(gradient: palette.backdropGradient),
               child: SafeArea(
                 child: Column(
                   children: [
@@ -1026,7 +1094,19 @@ class _NotesAppState extends State<NotesApp>
       title: 'Mis Notas',
       debugShowCheckedModeBanner: false,
       scaffoldMessengerKey: _scaffoldMessengerKey,
-      theme: _theme,
+      theme:
+          _lightTheme ??
+          AppTheme.theme(
+            seedColor: _themeSeedColor,
+            brightness: Brightness.light,
+          ),
+      darkTheme:
+          _darkTheme ??
+          AppTheme.theme(
+            seedColor: _themeSeedColor,
+            brightness: Brightness.dark,
+          ),
+      themeMode: _themeMode,
       home: homeWidget,
     );
   }