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/lang/en/messages.php

@@ -0,0 +1,117 @@
+<?php
+
+return [
+    // Navigation
+    'nav' => [
+        'dashboard' => 'Dashboard',
+        'knowledge_base' => 'Knowledge Base',
+        'profile' => 'Profile',
+        'user_management' => 'User Management',
+        'logout' => 'Log Out',
+        'login' => 'Login',
+        'register' => 'Register',
+    ],
+
+    // Documents
+    'documents' => [
+        'title' => 'Documents',
+        'new_document' => 'New Document',
+        'edit_document' => 'Edit Document',
+        'edit' => 'Edit',
+        'delete' => 'Delete',
+        'save' => 'Save',
+        'cancel' => 'Cancel',
+        'created_by' => 'Created by',
+        'updated' => 'Updated',
+        'path' => 'Path',
+        'last_modified' => 'Last modified',
+        'no_documents' => 'No documents found',
+        'search_placeholder' => 'Search documents...',
+        'create_success' => 'Document created successfully!',
+        'update_success' => 'Document updated successfully!',
+        'delete_success' => 'Document deleted successfully!',
+        'delete_confirm' => 'Are you sure you want to delete this document?',
+        'linked_references' => 'Linked References',
+        'title_label' => 'Title',
+        'title_placeholder' => 'Document Title (use / for folders, e.g. Laravel/Livewire/Components)',
+        'title_hint' => 'Tip: Use slashes (/) in the title to organize documents into folders automatically',
+        'content_label' => 'Content',
+        'content_placeholder' => 'Write your markdown here...',
+        'saving' => 'Saving...',
+    ],
+
+    // Quick Switcher
+    'quick_switcher' => [
+        'title' => 'Quick Switch',
+        'placeholder' => 'Search documents...',
+        'no_results' => 'No documents found',
+        'navigate' => 'to navigate',
+        'select' => 'to select',
+        'close' => 'to close',
+    ],
+
+    // Admin
+    'admin' => [
+        'user_management' => 'User Management',
+        'new_user' => 'New User',
+        'edit_user' => 'Edit User',
+        'create_user' => 'Create User',
+        'users' => 'Users',
+        'name' => 'Name',
+        'email' => 'Email',
+        'password' => 'Password',
+        'password_confirmation' => 'Confirm Password',
+        'password_hint' => 'Leave blank to keep current password.',
+        'role' => 'Role',
+        'admin' => 'Administrator',
+        'user' => 'User',
+        'grant_admin' => 'Grant administrator privileges',
+        'created_at' => 'Created',
+        'actions' => 'Actions',
+        'edit' => 'Edit',
+        'delete' => 'Delete',
+        'no_users' => 'No users found.',
+        'create_success' => 'User created successfully.',
+        'update_success' => 'User updated successfully.',
+        'delete_success' => 'User deleted successfully.',
+        'cannot_delete_self' => 'You cannot delete yourself.',
+        'self_admin_warning' => 'Removing your own admin privileges will lock you out of the admin panel.',
+    ],
+
+    // Settings
+    'settings' => [
+        'language' => 'Language',
+        'select_language' => 'Select Language',
+        'language_updated' => 'Language updated successfully.',
+    ],
+
+    // Common
+    'common' => [
+        'save' => 'Save',
+        'cancel' => 'Cancel',
+        'delete' => 'Delete',
+        'edit' => 'Edit',
+        'create' => 'Create',
+        'update' => 'Update',
+        'back' => 'Back',
+        'confirm' => 'Confirm',
+        'yes' => 'Yes',
+        'no' => 'No',
+        'loading' => 'Loading...',
+        'error' => 'Error',
+        'success' => 'Success',
+    ],
+
+    // Auth
+    'auth' => [
+        'login' => 'Login',
+        'register' => 'Register',
+        'email' => 'Email',
+        'password' => 'Password',
+        'remember_me' => 'Remember me',
+        'forgot_password' => 'Forgot your password?',
+        'confirm_password' => 'Confirm Password',
+        'already_registered' => 'Already registered?',
+    ],
+];
+