refactor: Update color handling to use ARGB format and improve theme consistency

lib/app.dart

@@ -212,7 +212,7 @@ class _NotesAppState extends State<NotesApp>
 
   Future<void> _setThemeSeedColor(Color color) async {
     final SharedPreferences prefs = await SharedPreferences.getInstance();
-    await prefs.setInt(_themeSeedColorKey, color.value);
+    await prefs.setInt(_themeSeedColorKey, color.toARGB32());
 
     if (!mounted) {
       return;

lib/screens/settings_screen.dart

@@ -361,7 +361,7 @@ class _SettingsScreenState extends State<SettingsScreen> {
 
   Widget _buildThemeColorButton(Color color) {
     final AppPalette palette = Theme.of(context).extension<AppPalette>()!;
-    final bool isSelected = _selectedSeedColor.value == color.value;
+    final bool isSelected = _selectedSeedColor.toARGB32() == color.toARGB32();
     final Color foregroundColor =
         ThemeData.estimateBrightnessForColor(color) == Brightness.dark
         ? palette.textPrimary
@@ -370,7 +370,7 @@ class _SettingsScreenState extends State<SettingsScreen> {
     return Semantics(
       button: true,
       selected: isSelected,
-      label: 'Color ${color.value.toRadixString(16)}',
+      label: 'Color ${color.toARGB32().toRadixString(16)}',
       child: Tooltip(
         message: isSelected ? 'Color actual' : 'Usar este color',
         child: InkWell(
@@ -490,6 +490,7 @@ class _SettingsScreenState extends State<SettingsScreen> {
     );
   }
 
+  @override
   Widget build(BuildContext context) {
     final AppPalette palette = Theme.of(context).extension<AppPalette>()!;
 

lib/theme/app_theme.dart

@@ -24,7 +24,7 @@ class AppTheme {
 
     return ThemeData(
       useMaterial3: true,
-      scaffoldBackgroundColor: scheme.background,
+      scaffoldBackgroundColor: scheme.surface,
       colorScheme: scheme,
       extensions: <ThemeExtension<dynamic>>[palette],
       brightness: brightness,

lib/widgets/menu_drawer.dart

@@ -61,10 +61,10 @@ class MenuDrawer extends StatelessWidget {
                                 ? null
                                 : () => onEditCategory?.call(category),
                             iconColor: Color(
-                              category.colorValue ?? palette.accent.value,
+                              category.colorValue ?? palette.accent.toARGB32(),
                             ),
                             textColor: Color(
-                              category.colorValue ?? palette.accent.value,
+                              category.colorValue ?? palette.accent.toARGB32(),
                             ),
                             trailing: IconButton(
                               padding: const EdgeInsets.all(8),

lib/widgets/search_app_bar.dart

@@ -94,7 +94,7 @@ class _SearchAppBarState extends State<SearchAppBar> {
                         decoration: InputDecoration(
                           hintText: widget.searchHint,
                           hintStyle: TextStyle(
-                            color: palette.textSecondary.withOpacity(0.6),
+                            color: palette.textSecondary.withValues(alpha: 0.6),
                           ),
                           suffixIcon: _searchController.text.isNotEmpty
                               ? IconButton(