feat: Mail template management UI and API CRUD

- Added full CRUD endpoints for mail templates (create, update, delete, preview)
- Introduced Joi validators for template create/update/preview
- Updated routes/controller to support ID and type lookups
- Built React Templates page with HTML editor, preview, and clipboard helpers
- Added navigation entry and route for /templates
- Enhanced documentation (README, QUICKSTART, KULLANIM, frontend/backend README)

frontend/src/components/Layout/Layout.jsx

@@ -25,6 +25,7 @@ import {
   Settings,
   Logout,
   AccountCircle,
+  Email,
 } from '@mui/icons-material';
 import { useAuth } from '../../context/AuthContext';
 
@@ -34,6 +35,7 @@ const menuItems = [
   { text: 'Dashboard', icon: <Dashboard />, path: '/' },
   { text: 'Şirketler', icon: <Business />, path: '/companies' },
   { text: 'Tokenlar', icon: <TokenIcon />, path: '/tokens' },
+  { text: 'Mail Şablonları', icon: <Email />, path: '/templates' },
   { text: 'Ayarlar', icon: <Settings />, path: '/settings' },
 ];