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/validators/template.validator.js

@@ -0,0 +1,63 @@
+const Joi = require('joi');
+
+const baseSchema = {
+  name: Joi.string().max(255).required(),
+  template_type: Joi.string().max(50).required(),
+  subject_template: Joi.string().max(500).allow('', null),
+  body_html: Joi.string().required(),
+  description: Joi.string().allow('', null),
+  active: Joi.boolean().default(true),
+};
+
+const createSchema = Joi.object(baseSchema);
+
+const updateSchema = Joi.object({
+  ...baseSchema,
+});
+
+const previewSchema = Joi.object({
+  template_html: Joi.string().required(),
+  company_name: Joi.string().allow('', null),
+  employee_name: Joi.string().allow('', null),
+});
+
+exports.validateCreateTemplate = (req, res, next) => {
+  const { error, value } = createSchema.validate(req.body, { abortEarly: false });
+  if (error) {
+    return res.status(400).json({
+      success: false,
+      error: 'Validation failed',
+      details: error.details.map((detail) => detail.message),
+    });
+  }
+  req.body = value;
+  next();
+};
+
+exports.validateUpdateTemplate = (req, res, next) => {
+  const { error, value } = updateSchema.validate(req.body, { abortEarly: false });
+  if (error) {
+    return res.status(400).json({
+      success: false,
+      error: 'Validation failed',
+      details: error.details.map((detail) => detail.message),
+    });
+  }
+  req.body = value;
+  next();
+};
+
+exports.validatePreviewTemplate = (req, res, next) => {
+  const { error, value } = previewSchema.validate(req.body, { abortEarly: false });
+  if (error) {
+    return res.status(400).json({
+      success: false,
+      error: 'Validation failed',
+      details: error.details.map((detail) => detail.message),
+    });
+  }
+  req.body = value;
+  next();
+};
+
+