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'));
+    }
+}