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.

test/widget_test.dart

@@ -8,6 +8,6 @@ void main() {
     await tester.pumpWidget(const NotesApp());
 
     expect(find.byType(MaterialApp), findsOneWidget);
-    expect(find.text('Mis Notas'), findsWidgets);
+    expect(find.text('Preparando el vault local...'), findsWidgets);
   });
 }