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/resources/views/layouts/navigation.blade.php

@@ -13,13 +13,13 @@
                 <!-- Navigation Links -->
                 <div class="hidden space-x-8 sm:-my-px sm:ms-10 sm:flex">
                     <x-nav-link :href="route('dashboard')" :active="request()->routeIs('dashboard')">
-                        {{ __('Dashboard') }}
+                        {{ __('messages.nav.dashboard') }}
                     </x-nav-link>
                     <x-nav-link href="/" :active="false">
                         <svg class="w-4 h-4 mr-1 inline" fill="none" stroke="currentColor" viewBox="0 0 24 24">
                             <path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M12 6.042A8.967 8.967 0 006 3.75c-1.052 0-2.062.18-3 .512v14.25A8.987 8.987 0 016 18c2.305 0 4.408.867 6 2.292m0-14.25a8.966 8.966 0 016-2.292c1.052 0 2.062.18 3 .512v14.25A8.987 8.987 0 0018 18a8.967 8.967 0 00-6 2.292m0-14.25v14.25"></path>
                         </svg>
-                        {{ __('Knowledge Base') }}
+                        {{ __('messages.nav.knowledge_base') }}
                     </x-nav-link>
                 </div>
             </div>
@@ -41,12 +41,12 @@
 
                     <x-slot name="content">
                         <x-dropdown-link :href="route('profile.edit')">
-                            {{ __('Profile') }}
+                            {{ __('messages.nav.profile') }}
                         </x-dropdown-link>
 
                         @if(Auth::user()->isAdmin())
                             <x-dropdown-link :href="route('admin.users.index')">
-                                {{ __('ユーザー管理') }}
+                                {{ __('messages.nav.user_management') }}
                             </x-dropdown-link>
                         @endif
 
@@ -57,7 +57,7 @@
                             <x-dropdown-link :href="route('logout')"
                                     onclick="event.preventDefault();
                                                 this.closest('form').submit();">
-                                {{ __('Log Out') }}
+                                {{ __('messages.nav.logout') }}
                             </x-dropdown-link>
                         </form>
                     </x-slot>
@@ -80,10 +80,10 @@
     <div :class="{'block': open, 'hidden': ! open}" class="hidden sm:hidden">
         <div class="pt-2 pb-3 space-y-1">
             <x-responsive-nav-link :href="route('dashboard')" :active="request()->routeIs('dashboard')">
-                {{ __('Dashboard') }}
+                {{ __('messages.nav.dashboard') }}
             </x-responsive-nav-link>
             <x-responsive-nav-link href="/" :active="false">
-                {{ __('Knowledge Base') }}
+                {{ __('messages.nav.knowledge_base') }}
             </x-responsive-nav-link>
         </div>
 
@@ -96,12 +96,12 @@
 
             <div class="mt-3 space-y-1">
                 <x-responsive-nav-link :href="route('profile.edit')">
-                    {{ __('Profile') }}
+                    {{ __('messages.nav.profile') }}
                 </x-responsive-nav-link>
 
                 @if(Auth::user()->isAdmin())
                     <x-responsive-nav-link :href="route('admin.users.index')">
-                        {{ __('ユーザー管理') }}
+                        {{ __('messages.nav.user_management') }}
                     </x-responsive-nav-link>
                 @endif
 
@@ -112,7 +112,7 @@
                     <x-responsive-nav-link :href="route('logout')"
                             onclick="event.preventDefault();
                                         this.closest('form').submit();">
-                        {{ __('Log Out') }}
+                        {{ __('messages.nav.logout') }}
                     </x-responsive-nav-link>
                 </form>
             </div>