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/screens/biometric_gate_screen.dart

@@ -1,5 +1,5 @@
 import 'package:flutter/material.dart';
-import 'package:notas/theme/app_colors.dart';
+import 'package:notas/theme/app_palette.dart';
 
 class BiometricGateScreen extends StatefulWidget {
   const BiometricGateScreen({
@@ -39,9 +39,11 @@ class _BiometricGateScreenState extends State<BiometricGateScreen> {
 
   @override
   Widget build(BuildContext context) {
+    final AppPalette palette = Theme.of(context).extension<AppPalette>()!;
+
     return Scaffold(
       body: Container(
-        decoration: const BoxDecoration(gradient: AppColors.backdropGradient),
+        decoration: BoxDecoration(gradient: palette.backdropGradient),
         child: SafeArea(
           child: Column(
             children: [
@@ -54,12 +56,12 @@ class _BiometricGateScreenState extends State<BiometricGateScreen> {
                       child: Container(
                         padding: const EdgeInsets.all(24),
                         decoration: BoxDecoration(
-                          color: AppColors.surface,
+                          color: palette.cardBackground,
                           borderRadius: BorderRadius.circular(24),
-                          border: Border.all(color: AppColors.borderMuted),
+                          border: Border.all(color: palette.border),
                           boxShadow: [
                             BoxShadow(
-                              color: AppColors.shadow,
+                              color: palette.shadowSoft,
                               blurRadius: 30,
                               offset: const Offset(0, 18),
                             ),
@@ -79,7 +81,7 @@ class _BiometricGateScreenState extends State<BiometricGateScreen> {
                               'Desbloqueo biométrico',
                               textAlign: TextAlign.center,
                               style: TextStyle(
-                                color: AppColors.textPrimary,
+                                color: Colors.white,
                                 fontSize: 28,
                                 fontWeight: FontWeight.w700,
                               ),
@@ -89,7 +91,7 @@ class _BiometricGateScreenState extends State<BiometricGateScreen> {
                               'Pon tu huella o cara para entrar a tus notas.',
                               textAlign: TextAlign.center,
                               style: TextStyle(
-                                color: AppColors.textSecondary,
+                                color: palette.textSecondary,
                                 height: 1.4,
                               ),
                             ),