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/category_style.dart

@@ -1,18 +1,10 @@
 import 'package:flutter/material.dart';
+import 'package:notas/theme/app_colors.dart';
 
 class CategoryStyle {
   CategoryStyle._();
 
-  static const List<Color> colors = <Color>[
-    Colors.amber,
-    Colors.blue,
-    Colors.green,
-    Colors.purple,
-    Colors.red,
-    Colors.teal,
-    Colors.orange,
-    Colors.grey,
-  ];
+  static const List<Color> colors = AppColors.categoryColors;
 
   static const List<IconData> icons = <IconData>[
     Icons.label_outline_rounded,
@@ -38,4 +30,4 @@ class CategoryStyle {
 
     return Icons.folder_outlined;
   }
-}
+}