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/widgets/note_card.dart

@@ -1,7 +1,7 @@
 import 'package:flutter/material.dart';
 
 import 'package:notas/models/note.dart';
-import 'package:notas/theme/app_colors.dart';
+import 'package:notas/theme/app_palette.dart';
 
 // Small presentational widget for a note inside the grid.
 // Keep this widget lightweight and layout-agnostic: it should not force
@@ -31,6 +31,7 @@ class _NoteCardState extends State<NoteCard> {
 
   @override
   Widget build(BuildContext context) {
+    final AppPalette palette = Theme.of(context).extension<AppPalette>()!;
     final bool showGrabbing = widget.isDragging || _isPressed;
 
     return MouseRegion(
@@ -63,10 +64,10 @@ class _NoteCardState extends State<NoteCard> {
         child: Container(
           padding: const EdgeInsets.all(16),
           decoration: BoxDecoration(
-            color: AppColors.cardBackground,
+            color: palette.cardBackground,
             borderRadius: BorderRadius.circular(12),
             border: Border.all(
-              color: widget.borderColor ?? AppColors.textDisabled,
+              color: widget.borderColor ?? palette.textDisabled,
               width: 1,
             ),
           ),
@@ -90,8 +91,8 @@ class _NoteCardState extends State<NoteCard> {
                 final TextPainter textPainter = TextPainter(
                   text: TextSpan(
                     text: widget.note.body,
-                    style: const TextStyle(
-                      color: AppColors.textSecondary,
+                    style: TextStyle(
+                      color: palette.textSecondary,
                       fontSize: 14,
                     ),
                   ),
@@ -110,8 +111,8 @@ class _NoteCardState extends State<NoteCard> {
                 children: [
                   Text(
                     widget.note.title,
-                    style: const TextStyle(
-                      color: AppColors.textPrimary,
+                    style: TextStyle(
+                      color: palette.textPrimary,
                       fontSize: 16,
                       fontWeight: FontWeight.bold,
                     ),
@@ -121,8 +122,8 @@ class _NoteCardState extends State<NoteCard> {
                   const SizedBox(height: 8),
                   Text(
                     widget.note.body,
-                    style: const TextStyle(
-                      color: AppColors.textSecondary,
+                    style: TextStyle(
+                      color: palette.textSecondary,
                       fontSize: 14,
                     ),
                     maxLines: 20,
@@ -130,10 +131,10 @@ class _NoteCardState extends State<NoteCard> {
                   ),
                   if (isBodyTruncated) ...[
                     const SizedBox(height: 4),
-                    const Text(
+                    Text(
                       '...',
                       style: TextStyle(
-                        color: AppColors.textMuted,
+                        color: palette.textMuted,
                         fontSize: 18,
                         height: 1,
                       ),