feat(frontend): add multi-language support (en, zh-Hans, zh-Hant, ja)

frontend/lib/pages/login_page.dart

@@ -1,6 +1,7 @@
 import 'dart:convert';
 import 'package:flutter/material.dart';
 import 'package:provider/provider.dart';
+import '../l10n/app_localizations.dart';
 import '../providers/auth_provider.dart';
 import '../providers/settings_provider.dart';
 import '../services/api_service.dart';
@@ -29,10 +30,11 @@ class _LoginPageState extends State<LoginPage> {
   }
 
   void _handleLogin() async {
+    final l10n = AppLocalizations.of(context)!;
     if (_usernameController.text.isEmpty || _passwordController.text.isEmpty) {
       ScaffoldMessenger.of(
         context,
-      ).showSnackBar(SnackBar(content: Text("Please fill in all fields")));
+      ).showSnackBar(SnackBar(content: Text(l10n.fillAllFields)));
       return;
     }
 
@@ -53,7 +55,7 @@ class _LoginPageState extends State<LoginPage> {
         if (!mounted) {
           return;
         }
-        final error = jsonDecode(response.body)['error'] ?? "Login Failed";
+        final error = jsonDecode(response.body)['error'] ?? l10n.loginFailed;
         ScaffoldMessenger.of(
           context,
         ).showSnackBar(SnackBar(content: Text(error)));
@@ -63,7 +65,7 @@ class _LoginPageState extends State<LoginPage> {
         return;
       }
       ScaffoldMessenger.of(context).showSnackBar(
-        SnackBar(content: Text("Network Error: Could not connect to server")),
+        SnackBar(content: Text(l10n.networkError)),
       );
     } finally {
       if (mounted) setState(() => _isLoading = false);
@@ -72,11 +74,13 @@ class _LoginPageState extends State<LoginPage> {
 
   @override
   Widget build(BuildContext context) {
+    final l10n = AppLocalizations.of(context)!;
+
     return Scaffold(
       appBar: AppBar(
         actions: [
           IconButton(
-            icon: Icon(Icons.settings),
+            icon: const Icon(Icons.settings),
             onPressed: () => Navigator.push(
               context,
               MaterialPageRoute(builder: (_) => const SettingsPage()),
@@ -88,7 +92,7 @@ class _LoginPageState extends State<LoginPage> {
         child: SingleChildScrollView(
           padding: const EdgeInsets.symmetric(horizontal: 32.0),
           child: ConstrainedBox(
-            constraints: BoxConstraints(maxWidth: 400),
+            constraints: const BoxConstraints(maxWidth: 400),
             child: Column(
               mainAxisAlignment: MainAxisAlignment.center,
               children: [
@@ -98,7 +102,7 @@ class _LoginPageState extends State<LoginPage> {
                   size: 80,
                   color: Theme.of(context).colorScheme.primary,
                 ),
-                SizedBox(height: 16),
+                const SizedBox(height: 16),
                 Text(
                   "HIGHTUBE",
                   style: TextStyle(
@@ -108,11 +112,11 @@ class _LoginPageState extends State<LoginPage> {
                     color: Theme.of(context).colorScheme.primary,
                   ),
                 ),
-                Text(
+                const Text(
                   "Open Source Live Platform",
                   style: TextStyle(color: Colors.grey),
                 ),
-                SizedBox(height: 48),
+                const SizedBox(height: 48),
 
                 // Fields
                 TextField(
@@ -120,14 +124,14 @@ class _LoginPageState extends State<LoginPage> {
                   textInputAction: TextInputAction.next,
                   onSubmitted: (_) => _passwordFocusNode.requestFocus(),
                   decoration: InputDecoration(
-                    labelText: "Username",
-                    prefixIcon: Icon(Icons.person),
+                    labelText: l10n.username,
+                    prefixIcon: const Icon(Icons.person),
                     border: OutlineInputBorder(
                       borderRadius: BorderRadius.circular(12),
                     ),
                   ),
                 ),
-                SizedBox(height: 16),
+                const SizedBox(height: 16),
                 TextField(
                   controller: _passwordController,
                   focusNode: _passwordFocusNode,
@@ -139,14 +143,14 @@ class _LoginPageState extends State<LoginPage> {
                     }
                   },
                   decoration: InputDecoration(
-                    labelText: "Password",
-                    prefixIcon: Icon(Icons.lock),
+                    labelText: l10n.password,
+                    prefixIcon: const Icon(Icons.lock),
                     border: OutlineInputBorder(
                       borderRadius: BorderRadius.circular(12),
                     ),
                   ),
                 ),
-                SizedBox(height: 32),
+                const SizedBox(height: 32),
 
                 // Login Button
                 SizedBox(
@@ -160,14 +164,14 @@ class _LoginPageState extends State<LoginPage> {
                       ),
                     ),
                     child: _isLoading
-                        ? CircularProgressIndicator()
+                        ? const CircularProgressIndicator()
                         : Text(
-                            "LOGIN",
-                            style: TextStyle(fontWeight: FontWeight.bold),
+                            l10n.login,
+                            style: const TextStyle(fontWeight: FontWeight.bold),
                           ),
                   ),
                 ),
-                SizedBox(height: 16),
+                const SizedBox(height: 16),
 
                 // Register Link
                 TextButton(
@@ -175,7 +179,7 @@ class _LoginPageState extends State<LoginPage> {
                     context,
                     MaterialPageRoute(builder: (_) => RegisterPage()),
                   ),
-                  child: Text("Don't have an account? Create one"),
+                  child: Text(l10n.dontHaveAccount),
                 ),
               ],
             ),