feat: Implement note encryption and synchronization features

- Added NoteEncryption class for encrypting and decrypting note content using AES-GCM.
- Updated NoteRepository to handle synchronization of notes and categories with the server, including encryption of note data before sending.
- Introduced SyncRequest and SyncResponse models for managing synchronization data.
- Enhanced LocalVaultService to store and retrieve the encryption key.
- Modified HomeScreen and SettingsScreen to trigger synchronization after note operations and manage API endpoint settings.
- Added SyncStatusIndicator to provide visual feedback on synchronization status in the app title bar.
- Created Category model to manage note categories with encryption support.
- Updated note model to include UUID, server version, deletion status, and category ID.
- Added necessary UI elements for displaying and managing the encryption key in SettingsScreen.
- Updated dependencies in pubspec.yaml for cryptography and HTTP handling.

lib/screens/vault_access_screen.dart

@@ -1,5 +1,6 @@
 import 'package:flutter/material.dart';
 import 'package:notas/widgets/app_title_bar.dart';
+import 'package:notas/data/api_client.dart';
 
 class VaultAccessScreen extends StatefulWidget {
   const VaultAccessScreen({
@@ -22,6 +23,29 @@ class VaultAccessScreen extends StatefulWidget {
 class _VaultAccessScreenState extends State<VaultAccessScreen> {
   final TextEditingController _emailController = TextEditingController();
   final TextEditingController _passwordController = TextEditingController();
+  final TextEditingController _endpointController = TextEditingController();
+  bool _endpointLoading = true;
+
+  @override
+  void initState() {
+    super.initState();
+    _loadEndpoint();
+  }
+
+  Future<void> _loadEndpoint() async {
+    final String endpoint = await ApiConfig.getEndpoint();
+    if (!mounted) return;
+    _endpointController.text = endpoint;
+    setState(() {
+      _endpointLoading = false;
+    });
+  }
+
+  Future<void> _persistEndpointFromField() async {
+    final String value = _endpointController.text.trim();
+    if (value.isEmpty) return;
+    await ApiConfig.setEndpoint(value);
+  }
 
   @override
   void dispose() {
@@ -31,6 +55,8 @@ class _VaultAccessScreenState extends State<VaultAccessScreen> {
   }
 
   Future<void> _handleCreateAccount() async {
+    await _persistEndpointFromField();
+
     await widget.onCreateAccountPressed(
       _emailController.text.trim(),
       _passwordController.text,
@@ -38,6 +64,8 @@ class _VaultAccessScreenState extends State<VaultAccessScreen> {
   }
 
   Future<void> _handleSignIn() async {
+    await _persistEndpointFromField();
+
     await widget.onSignInPressed(
       _emailController.text.trim(),
       _passwordController.text,
@@ -112,13 +140,43 @@ class _VaultAccessScreenState extends State<VaultAccessScreen> {
                               ),
                             ),
                             const SizedBox(height: 28),
+                            _endpointLoading
+                                ? const SizedBox(
+                                    height: 48,
+                                    child: Center(child: CircularProgressIndicator()),
+                                  )
+                                : TextField(
+                                    controller: _endpointController,
+                                    enabled: !widget.isBusy,
+                                    keyboardType: TextInputType.url,
+                                    style: const TextStyle(color: Colors.white),
+                                    decoration: InputDecoration(
+                                      labelText: 'API endpoint',
+                                      labelStyle: TextStyle(color: Colors.white.withValues(alpha: 0.7)),
+                                      filled: true,
+                                      fillColor: Colors.white.withValues(alpha: 0.05),
+                                      border: OutlineInputBorder(
+                                        borderRadius: BorderRadius.circular(14),
+                                        borderSide: BorderSide(color: Colors.white.withValues(alpha: 0.12)),
+                                      ),
+                                      enabledBorder: OutlineInputBorder(
+                                        borderRadius: BorderRadius.circular(14),
+                                        borderSide: BorderSide(color: Colors.white.withValues(alpha: 0.12)),
+                                      ),
+                                      focusedBorder: OutlineInputBorder(
+                                        borderRadius: BorderRadius.circular(14),
+                                        borderSide: const BorderSide(color: Colors.amber, width: 1.2),
+                                      ),
+                                    ),
+                                  ),
+                            const SizedBox(height: 16),
                             TextField(
                               controller: _emailController,
                               enabled: !widget.isBusy,
-                              keyboardType: TextInputType.emailAddress,
+                              keyboardType: TextInputType.text,
                               style: const TextStyle(color: Colors.white),
                               decoration: InputDecoration(
-                                labelText: 'Email',
+                                labelText: 'Usuario',
                                 labelStyle: TextStyle(color: Colors.white.withValues(alpha: 0.7)),
                                 filled: true,
                                 fillColor: Colors.white.withValues(alpha: 0.05),