fix: credential data corruption - Create handler mutating store pointer

- CreateCredential was setting cred.Data = cred.Masked directly on the
  store pointer, corrupting stored credentials with masked data
- Fixed by using a value copy for API response
- Also fix nextID starts at 1 to avoid ID=0 issue

backend/config/credential_store.go

@@ -71,8 +71,9 @@ var CredStore *CredentialStore
 // InitCredentialStore initializes the credential store
 func InitCredentialStore(cfg *Config) {
 	CredStore = &CredentialStore{
-		data: make(map[uint]*Credential),
-		path: cfg.DataDir + "/credentials.json",
+		data:   make(map[uint]*Credential),
+		path:   cfg.DataDir + "/credentials.json",
+		nextID: 1,
 	}
 
 	if err := CredStore.Load(); err != nil {