Refactor theme colors and styles across the application

- Introduced AppColors class to centralize color definitions for better maintainability and consistency.
- Updated various screens (Settings, Vault Access, Note Card, etc.) to use AppColors for styling instead of hardcoded colors.
- Enhanced UI elements with improved color contrast and accessibility.
- Replaced gradient backgrounds with defined color schemes for a cohesive look.
- Refactored button styles and text colors to align with the new theme structure.

lib/widgets/note_card.dart

@@ -1,6 +1,7 @@
 import 'package:flutter/material.dart';
 
 import 'package:notas/models/note.dart';
+import 'package:notas/theme/app_colors.dart';
 
 // Small presentational widget for a note inside the grid.
 // Keep this widget lightweight and layout-agnostic: it should not force
@@ -33,7 +34,9 @@ class _NoteCardState extends State<NoteCard> {
     final bool showGrabbing = widget.isDragging || _isPressed;
 
     return MouseRegion(
-      cursor: showGrabbing ? SystemMouseCursors.grabbing : SystemMouseCursors.grab,
+      cursor: showGrabbing
+          ? SystemMouseCursors.grabbing
+          : SystemMouseCursors.grab,
       child: GestureDetector(
         onTapDown: widget.onTap == null
             ? null
@@ -60,10 +63,10 @@ class _NoteCardState extends State<NoteCard> {
         child: Container(
           padding: const EdgeInsets.all(16),
           decoration: BoxDecoration(
-            color: const Color.fromRGBO(24, 25, 26, 1),
+            color: AppColors.cardBackground,
             borderRadius: BorderRadius.circular(12),
             border: Border.all(
-              color: widget.borderColor ?? Colors.white24,
+              color: widget.borderColor ?? AppColors.textDisabled,
               width: 1,
             ),
           ),
@@ -87,7 +90,10 @@ class _NoteCardState extends State<NoteCard> {
                 final TextPainter textPainter = TextPainter(
                   text: TextSpan(
                     text: widget.note.body,
-                    style: const TextStyle(color: Colors.white70, fontSize: 14),
+                    style: const TextStyle(
+                      color: AppColors.textSecondary,
+                      fontSize: 14,
+                    ),
                   ),
                   maxLines: 20,
                   textDirection: TextDirection.ltr,
@@ -105,7 +111,7 @@ class _NoteCardState extends State<NoteCard> {
                   Text(
                     widget.note.title,
                     style: const TextStyle(
-                      color: Colors.white,
+                      color: AppColors.textPrimary,
                       fontSize: 16,
                       fontWeight: FontWeight.bold,
                     ),
@@ -115,7 +121,10 @@ class _NoteCardState extends State<NoteCard> {
                   const SizedBox(height: 8),
                   Text(
                     widget.note.body,
-                    style: const TextStyle(color: Colors.white70, fontSize: 14),
+                    style: const TextStyle(
+                      color: AppColors.textSecondary,
+                      fontSize: 14,
+                    ),
                     maxLines: 20,
                     overflow: TextOverflow.clip,
                   ),
@@ -124,7 +133,7 @@ class _NoteCardState extends State<NoteCard> {
                     const Text(
                       '...',
                       style: TextStyle(
-                        color: Colors.white54,
+                        color: AppColors.textMuted,
                         fontSize: 18,
                         height: 1,
                       ),
@@ -138,4 +147,4 @@ class _NoteCardState extends State<NoteCard> {
       ),
     );
   }
-}
+}