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/screens/biometric_gate_screen.dart

@@ -1,4 +1,5 @@
 import 'package:flutter/material.dart';
+import 'package:notas/theme/app_colors.dart';
 
 class BiometricGateScreen extends StatefulWidget {
   const BiometricGateScreen({
@@ -40,17 +41,7 @@ class _BiometricGateScreenState extends State<BiometricGateScreen> {
   Widget build(BuildContext context) {
     return Scaffold(
       body: Container(
-        decoration: const BoxDecoration(
-          gradient: LinearGradient(
-            colors: [
-              Color(0xFF191A1D),
-              Color(0xFF222326),
-              Color(0xFF101114),
-            ],
-            begin: Alignment.topLeft,
-            end: Alignment.bottomRight,
-          ),
-        ),
+        decoration: const BoxDecoration(gradient: AppColors.backdropGradient),
         child: SafeArea(
           child: Column(
             children: [
@@ -63,12 +54,12 @@ class _BiometricGateScreenState extends State<BiometricGateScreen> {
                       child: Container(
                         padding: const EdgeInsets.all(24),
                         decoration: BoxDecoration(
-                          color: const Color(0xFF1D1E20),
+                          color: AppColors.surface,
                           borderRadius: BorderRadius.circular(24),
-                          border: Border.all(color: Colors.white.withValues(alpha: 0.08)),
+                          border: Border.all(color: AppColors.borderMuted),
                           boxShadow: [
                             BoxShadow(
-                              color: Colors.black.withValues(alpha: 0.35),
+                              color: AppColors.shadow,
                               blurRadius: 30,
                               offset: const Offset(0, 18),
                             ),
@@ -88,7 +79,7 @@ class _BiometricGateScreenState extends State<BiometricGateScreen> {
                               'Desbloqueo biométrico',
                               textAlign: TextAlign.center,
                               style: TextStyle(
-                                color: Colors.white,
+                                color: AppColors.textPrimary,
                                 fontSize: 28,
                                 fontWeight: FontWeight.w700,
                               ),
@@ -98,21 +89,27 @@ class _BiometricGateScreenState extends State<BiometricGateScreen> {
                               'Pon tu huella o cara para entrar a tus notas.',
                               textAlign: TextAlign.center,
                               style: TextStyle(
-                                color: Colors.white.withValues(alpha: 0.72),
+                                color: AppColors.textSecondary,
                                 height: 1.4,
                               ),
                             ),
                             const SizedBox(height: 22),
                             FilledButton(
-                              onPressed: widget.isBusy ? null : widget.onUnlockRequested,
+                              onPressed: widget.isBusy
+                                  ? null
+                                  : widget.onUnlockRequested,
                               style: FilledButton.styleFrom(
-                                padding: const EdgeInsets.symmetric(vertical: 14),
+                                padding: const EdgeInsets.symmetric(
+                                  vertical: 14,
+                                ),
                               ),
                               child: widget.isBusy
                                   ? const SizedBox(
                                       width: 18,
                                       height: 18,
-                                      child: CircularProgressIndicator(strokeWidth: 2),
+                                      child: CircularProgressIndicator(
+                                        strokeWidth: 2,
+                                      ),
                                     )
                                   : const Text('Desbloquear'),
                             ),