feat(spotify): improve Spotify registration flow and speaker notification

- Implement full SoundTouch app flow for Spotify registration in the Web UI.
- Update `/mgmt/spotify/init` to pass `accountID` via OAuth `state`.
- Add "Connect Spotify" button to Local Account tab in Web UI with polling.
- Implement legacy and Marge-sync fallbacks for speaker notifications (Error 1029).
- Add support for parsing multi-error XML responses (`<errors>`) from speakers.
- Add `NotifySourcesUpdated` to client for triggering manual source synchronization.
- Improve test coverage for error parsing and Spotify initialization handlers.

Co-authored-by: Junie <junie@jetbrains.com>

pkg/models/account.go

@@ -141,7 +141,13 @@ type MusicServiceAccountResponse struct {
 	Status  string   `xml:",chardata"`
 }
 
+// SourcesUpdatedResponse represents the response from /notification
+type SourcesUpdatedResponse struct {
+	XMLName xml.Name `xml:"status"`
+	Status  string   `xml:",chardata"`
+}
+
 // IsSuccess returns true if the account operation was successful
 func (resp *MusicServiceAccountResponse) IsSuccess() bool {
-	return resp.Status == "/setMusicServiceAccount" || resp.Status == "/removeMusicServiceAccount"
+	return resp.Status == "/setMusicServiceAccount" || resp.Status == "/removeMusicServiceAccount" || resp.Status == "/notification"
 }

pkg/models/device.go

@@ -36,6 +36,22 @@ type NetworkInfo struct {
 	IPAddress  string `xml:"ipAddress"`
 }
 
+// SourcesUpdatedNotification represents the notification XML sent to the device
+type SourcesUpdatedNotification struct {
+	XMLName  xml.Name `xml:"updates"`
+	DeviceID string   `xml:"deviceID,attr"`
+	Sources  struct {
+		XMLName xml.Name `xml:"sourcesUpdated"`
+	} `xml:"sourcesUpdated"`
+}
+
+// NewSourcesUpdatedNotification creates a new sources updated notification
+func NewSourcesUpdatedNotification(deviceID string) *SourcesUpdatedNotification {
+	return &SourcesUpdatedNotification{
+		DeviceID: deviceID,
+	}
+}
+
 // XMLResponse is a generic wrapper for API responses
 type XMLResponse struct {
 	XMLName xml.Name
@@ -53,6 +69,29 @@ func (e *APIError) Error() string {
 	return e.Message
 }
 
+// ErrorsResponse represents a multi-error response from the API (common in some firmware versions)
+type ErrorsResponse struct {
+	XMLName  xml.Name      `xml:"errors"`
+	DeviceID string        `xml:"deviceID,attr"`
+	Errors   []DeviceError `xml:"error"`
+}
+
+// Error implements the error interface for ErrorsResponse
+func (e *ErrorsResponse) Error() string {
+	if len(e.Errors) > 0 {
+		return e.Errors[0].Message
+	}
+
+	return "unknown API error"
+}
+
+// DeviceError represents a single error in an ErrorsResponse
+type DeviceError struct {
+	Value   int    `xml:"value,attr"`
+	Name    string `xml:"name,attr"`
+	Message string `xml:",chardata"`
+}
+
 // DiscoveredDevice represents a device found through network discovery
 type DiscoveredDevice struct {
 	Name            string    `json:"name"`