fix: transactional env var save, empty key validation, frontend error handling

- Wrap DELETE + INSERTs in a database transaction via new
  ReplaceEnvVarsByAppID() to prevent silent data loss on partial
  insert failure. Rollback on any error; return 500 instead of 200.
- Add server-side validation rejecting entries with empty keys
  (returns 400 with error message).
- Add frontend error handling for non-2xx responses with user-visible
  alert messages.
- Remove stale //nolint:dupl directives (files no longer duplicate).

internal/models/label.go

@@ -1,4 +1,3 @@
-//nolint:dupl // Active Record pattern - similar structure to env_var.go is intentional
 package models
 
 import (