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)

backend/src/routes/template.routes.js

@@ -2,13 +2,22 @@ const express = require('express');
 const router = express.Router();
 const templateController = require('../controllers/template.controller');
 const { requireAuth } = require('../middlewares/auth');
+const {
+  validateCreateTemplate,
+  validateUpdateTemplate,
+  validatePreviewTemplate,
+} = require('../validators/template.validator');
 
 // All template routes require authentication
 router.use(requireAuth);
 
 router.get('/', templateController.getAllTemplates);
-router.get('/:type', templateController.getTemplateByType);
-router.post('/preview', templateController.previewTemplate);
+router.post('/', validateCreateTemplate, templateController.createTemplate);
+router.get('/:id(\\d+)', templateController.getTemplateById);
+router.put('/:id(\\d+)', validateUpdateTemplate, templateController.updateTemplate);
+router.delete('/:id(\\d+)', templateController.deleteTemplate);
+router.get('/type/:type', templateController.getTemplateByType);
+router.post('/preview', validatePreviewTemplate, templateController.previewTemplate);
 
 module.exports = router;