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/routes/web.php

@@ -1,6 +1,7 @@
 <?php
 
 use App\Http\Controllers\ProfileController;
+use App\Http\Controllers\LocaleController;
 use App\Http\Controllers\Admin\UserController as AdminUserController;
 use App\Livewire\DocumentViewer;
 use App\Livewire\DocumentEditor;
@@ -25,6 +26,7 @@
     Route::get('/profile', [ProfileController::class, 'edit'])->name('profile.edit');
     Route::patch('/profile', [ProfileController::class, 'update'])->name('profile.update');
     Route::delete('/profile', [ProfileController::class, 'destroy'])->name('profile.destroy');
+    Route::post('/locale', [LocaleController::class, 'update'])->name('locale.update');
 });
 
 // Admin routes