[REFACTOR] 實作側邊欄與儀表板多語系化，修復 UI 位移與樣式優化

routes/web.php

@@ -14,6 +14,9 @@ use Illuminate\Support\Facades\Route;
 |
 */
 
+// Multi-language switch
+Route::get('lang/{locale}', [App\Http\Controllers\System\LanguageController::class, 'switch'])->name('lang.switch');
+
 Route::get('/', function () {
     return redirect()->route('login');
 });