feat: Add multi-language support (i18n)

Languages supported (8):
- English (en)
- 日本語 (ja)
- Deutsch (de)
- Français (fr)
- Español (es)
- 简体中文 (zh-CN)
- 繁體中文 (zh-TW)
- 한국어 (ko)

Changes:
- Add locale column to users table
- Add SetLocale middleware for automatic locale detection
- Add LocaleController for language switching
- Create language files with translations for all UI elements
- Add language selector to user profile page
- Update all Blade views to use translation strings

src/app/Http/Controllers/LocaleController.php

@@ -0,0 +1,30 @@
+<?php
+
+namespace App\Http\Controllers;
+
+use App\Http\Middleware\SetLocale;
+use Illuminate\Http\Request;
+use Illuminate\Support\Facades\Auth;
+
+class LocaleController extends Controller
+{
+    /**
+     * Update the user's locale preference.
+     */
+    public function update(Request $request)
+    {
+        $validated = $request->validate([
+            'locale' => ['required', 'string', 'in:' . implode(',', array_keys(SetLocale::SUPPORTED_LOCALES))],
+        ]);
+
+        $locale = $validated['locale'];
+
+        // Save to user record
+        Auth::user()->update(['locale' => $locale]);
+
+        // Also save to session for immediate effect
+        $request->session()->put('locale', $locale);
+
+        return redirect()->route('profile.edit')->with('success', __('messages.settings.language_updated'));
+    }
+}

src/app/Http/Middleware/SetLocale.php

@@ -0,0 +1,54 @@
+<?php
+
+namespace App\Http\Middleware;
+
+use Closure;
+use Illuminate\Http\Request;
+use Illuminate\Support\Facades\App;
+use Illuminate\Support\Facades\Auth;
+use Symfony\Component\HttpFoundation\Response;
+
+class SetLocale
+{
+    /**
+     * Supported locales
+     */
+    public const SUPPORTED_LOCALES = [
+        'en' => 'English',
+        'ja' => '日本語',
+        'de' => 'Deutsch',
+        'fr' => 'Français',
+        'es' => 'Español',
+        'zh-CN' => '简体中文',
+        'zh-TW' => '繁體中文',
+        'ko' => '한국어',
+    ];
+
+    /**
+     * Handle an incoming request.
+     *
+     * @param  \Closure(\Illuminate\Http\Request): (\Symfony\Component\HttpFoundation\Response)  $next
+     */
+    public function handle(Request $request, Closure $next): Response
+    {
+        $locale = config('app.locale', 'en');
+
+        // Check authenticated user's preference
+        if (Auth::check() && Auth::user()->locale) {
+            $locale = Auth::user()->locale;
+        }
+        // Check session (for immediate effect after changing)
+        elseif ($request->session()->has('locale')) {
+            $locale = $request->session()->get('locale');
+        }
+
+        // Validate locale
+        if (!array_key_exists($locale, self::SUPPORTED_LOCALES)) {
+            $locale = 'en';
+        }
+
+        App::setLocale($locale);
+
+        return $next($request);
+    }
+}

src/app/Models/User.php

@@ -22,6 +22,7 @@ class User extends Authenticatable
         'email',
         'password',
         'is_admin',
+        'locale',
     ];
 
     /**