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/livewire/document-editor.blade.php

@@ -15,7 +15,7 @@
     <!-- Header -->
     <div class="mb-6 flex items-center justify-between">
         <h1 class="text-3xl font-bold text-gray-900">
-            {{ $isEditMode ? 'Edit Document' : 'New Document' }}
+            {{ $isEditMode ? __('messages.documents.edit_document') : __('messages.documents.new_document') }}
         </h1>
 
         <div class="flex space-x-3">
@@ -24,22 +24,22 @@
                     href="{{ route('documents.show', $document) }}"
                     class="inline-flex items-center px-4 py-2 border border-gray-300 rounded-md text-sm font-medium text-gray-700 bg-white hover:bg-gray-50"
                 >
-                    Cancel
+                    {{ __('messages.common.cancel') }}
                 </a>
 
                 <button
                     wire:click="delete"
-                    wire:confirm="Are you sure you want to delete this document?"
+                    wire:confirm="{{ __('messages.documents.delete_confirm') }}"
                     class="inline-flex items-center px-4 py-2 border border-red-300 rounded-md text-sm font-medium text-red-700 bg-white hover:bg-red-50"
                 >
-                    Delete
+                    {{ __('messages.documents.delete') }}
                 </button>
             @else
                 <a
                     href="{{ route('documents.show', 'home') }}"
                     class="inline-flex items-center px-4 py-2 border border-gray-300 rounded-md text-sm font-medium text-gray-700 bg-white hover:bg-gray-50"
                 >
-                    Cancel
+                    {{ __('messages.common.cancel') }}
                 </a>
             @endif
 
@@ -50,7 +50,7 @@ class="inline-flex items-center px-4 py-2 bg-indigo-600 border border-transparen
                 <svg class="w-4 h-4 mr-2" fill="none" stroke="currentColor" viewBox="0 0 24 24">
                     <path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M8 7H5a2 2 0 00-2 2v9a2 2 0 002 2h14a2 2 0 002-2V9a2 2 0 00-2-2h-3m-1 4l-3 3m0 0l-3-3m3 3V4"></path>
                 </svg>
-                Save
+                {{ __('messages.documents.save') }}
             </button>
         </div>
     </div>
@@ -60,27 +60,27 @@ class="inline-flex items-center px-4 py-2 bg-indigo-600 border border-transparen
         <!-- Title -->
         <div>
             <label for="title" class="block text-sm font-medium text-gray-700 mb-2">
-                Title
+                {{ __('messages.documents.title_label') }}
             </label>
             <input
                 type="text"
                 id="title"
                 wire:model="title"
                 class="w-full px-4 py-2 border border-gray-300 rounded-md focus:ring-2 focus:ring-indigo-500 focus:border-indigo-500"
-                placeholder="Document Title (use / for folders, e.g. Laravel/Livewire/Components)"
+                placeholder="{{ __('messages.documents.title_placeholder') }}"
             >
             @error('title')
                 <p class="mt-1 text-sm text-red-600">{{ $message }}</p>
             @enderror
             <p class="mt-1 text-xs text-gray-500">
-                Tip: Use slashes (/) in the title to organize documents into folders automatically
+                {{ __('messages.documents.title_hint') }}
             </p>
         </div>
 
         <!-- Markdown Editor -->
         <div>
             <label class="block text-sm font-medium text-gray-700 mb-2">
-                Content
+                {{ __('messages.documents.content_label') }}
             </label>
             <div wire:ignore>
                 <div x-data="markdownEditor()" x-init="initEditor()">
@@ -103,12 +103,12 @@ class="w-full"
                     <circle class="opacity-25" cx="12" cy="12" r="10" stroke="currentColor" stroke-width="4"></circle>
                     <path class="opacity-75" fill="currentColor" d="M4 12a8 8 0 018-8V0C5.373 0 0 5.373 0 12h4zm2 5.291A7.962 7.962 0 014 12H0c0 3.042 1.135 5.824 3 7.938l3-2.647z"></path>
                 </svg>
-                Saving...
+                {{ __('messages.documents.saving') }}
             </div>
 
             @if($isEditMode && $document)
                 <div>
-                    Path: <code class="text-xs bg-gray-100 px-2 py-1 rounded">{{ $document->path }}</code>
+                    {{ __('messages.documents.path') }}: <code class="text-xs bg-gray-100 px-2 py-1 rounded">{{ $document->path }}</code>
                 </div>
             @endif
         </div>
@@ -137,7 +137,7 @@ class="w-full"
                         autosave: {
                             enabled: false,
                         },
-                        placeholder: 'Write your markdown here...',
+                        placeholder: '{{ __("messages.documents.content_placeholder") }}',
                         toolbar: [
                             'bold', 'italic', 'heading', '|',
                             'quote', 'unordered-list', 'ordered-list', '|',
@@ -148,9 +148,7 @@ class="w-full"
                         status: ['lines', 'words', 'cursor'],
                     });
 
-                    // エディタの変更をLivewireに反映
                     this.editor.codemirror.on('change', () => {
-                        // wire:ignoreの外にあるhidden inputを探す
                         const formElement = this.$el.closest('form');
                         const hiddenInput = formElement.querySelector('input[type="hidden"][wire\\:model="content"]');
                         if (hiddenInput) {