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

@@ -73,6 +73,7 @@ func InitCredentialStore(cfg *Config) {
 	CredStore = &CredentialStore{
 		data:   make(map[uint]*Credential),
 		path:   cfg.DataDir + "/credentials.json",
+		nextID: 1,
 	}
 
 	if err := CredStore.Load(); err != nil {

backend/handlers/credential.go

@@ -89,9 +89,10 @@ func (h *CredentialHandler) CreateCredential(c *gin.Context) {
 		return
 	}
 
-	// Return without secrets
-	cred.Data = cred.Masked
-	c.JSON(http.StatusCreated, cred)
+	// Return without secrets (use a copy to avoid mutating store)
+	resp := *cred
+	resp.Data = cred.Masked
+	c.JSON(http.StatusCreated, &resp)
 }
 
 // UpdateCredential updates a credential