fix(sync): add ClientSequence and LastSeenServerSeq to Op struct and Push

- Add ClientSequence and LastSeenServerSeq fields to sync.Op struct
- Fix Client.Push() to copy these fields to PushOp
- Add GetDeviceID() method to sync.Service

.gitignore

@@ -25,6 +25,7 @@ frontend/bindings/
 /verstak-gui
 /verstak-cli
 /verstak-server
+/verstak
 
 # Vault data
 .verstak/

internal/core/sync/client.go

@@ -130,6 +130,19 @@ func (c *Client) Login(username, password string) (token string, err error) {
 	return resp.Token, nil
 }
 
+// TestAuth checks credentials without creating a device.
+func (c *Client) TestAuth(serverURL, username, password string) error {
+	body := map[string]string{"username": username, "password": password}
+	savedURL := c.ServerURL
+	savedKey := c.APIKey
+	c.ServerURL = serverURL
+	c.APIKey = ""
+	err := c.post("/api/auth/test", body, nil)
+	c.ServerURL = savedURL
+	c.APIKey = savedKey
+	return err
+}
+
 // PushRequest is the payload for POST /sync/push.
 type PushRequest struct {
 	DeviceID      string   `json:"device_id"`
@@ -166,6 +179,8 @@ func (c *Client) Push(ops []Op) (*PushResponse, error) {
 			EntityID:          op.EntityID,
 			OpType:            op.OpType,
 			PayloadJSON:       op.PayloadJSON,
+			ClientSequence:    op.ClientSequence,
+			LastSeenServerSeq: op.LastSeenServerSeq,
 			CreatedAt:         op.CreatedAt,
 		}
 	}

internal/core/sync/sync.go

@@ -40,6 +40,8 @@ type Op struct {
 	PayloadJSON       string   `json:"payload_json"`
 	CreatedAt         string   `json:"created_at"`
 	PushedAt          *string  `json:"pushed_at,omitempty"`
+	ClientSequence    int      `json:"client_sequence,omitempty"`
+	LastSeenServerSeq int      `json:"last_seen_server_seq,omitempty"`
 }
 
 // Service records and manages sync operations.