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/theme/app_theme.dart

@@ -1,21 +1,33 @@
 import 'package:flutter/material.dart';
-import 'package:notas/theme/app_colors.dart';
+import 'package:notas/theme/app_palette.dart';
 
 class AppTheme {
-  static ThemeData theme({Color seedColor = AppColors.defaultThemeSeedColor}) {
+  static ThemeData theme({
+    Color seedColor = Colors.amber,
+    Brightness brightness = Brightness.dark,
+  }) {
     final Brightness foregroundBrightness =
         ThemeData.estimateBrightnessForColor(seedColor);
     final Color foregroundColor = foregroundBrightness == Brightness.dark
-        ? AppColors.textPrimary
-        : AppColors.textOnAccent;
+        ? Colors.white
+        : Colors.black87;
+
+    final ColorScheme scheme = ColorScheme.fromSeed(
+      seedColor: seedColor,
+      brightness: brightness,
+    );
+
+    final AppPalette palette = AppPalette.fromBrightness(
+      brightness,
+      seedColor: seedColor,
+    );
 
     return ThemeData(
       useMaterial3: true,
-      scaffoldBackgroundColor: AppColors.scaffoldBackground,
-      colorScheme: ColorScheme.fromSeed(
-        seedColor: seedColor,
-        brightness: Brightness.dark,
-      ),
+      scaffoldBackgroundColor: scheme.background,
+      colorScheme: scheme,
+      extensions: <ThemeExtension<dynamic>>[palette],
+      brightness: brightness,
       floatingActionButtonTheme: FloatingActionButtonThemeData(
         backgroundColor: seedColor,
         foregroundColor: foregroundColor,