fix: include genre, label, copyright in DownloadResponse

Extended metadata was being embedded into FLAC files but not returned
in the response to Flutter, causing history to not store these fields.

Fixed in 3 places in extension_providers.go:
- Source extension download response
- Extension fallback download response
- Built-in provider (Tidal/Qobuz/Amazon) response

go_backend/extension_providers.go

@@ -848,6 +848,9 @@ func DownloadWithExtensionFallback(req DownloadRequest) (*DownloadResponse, erro
 					ActualBitDepth:   result.BitDepth,
 					ActualSampleRate: result.SampleRate,
 					Service:          req.Source,
+					Genre:            req.Genre,
+					Label:            req.Label,
+					Copyright:        req.Copyright,
 				}
 
 				// Embed genre and label if provided (from Deezer metadata)
@@ -1010,6 +1013,9 @@ func DownloadWithExtensionFallback(req DownloadRequest) (*DownloadResponse, erro
 					ActualBitDepth:   result.BitDepth,
 					ActualSampleRate: result.SampleRate,
 					Service:          providerID,
+					Genre:            req.Genre,
+					Label:            req.Label,
+					Copyright:        req.Copyright,
 				}
 
 				// Embed genre and label if provided (from Deezer metadata)
@@ -1169,6 +1175,9 @@ func tryBuiltInProvider(providerID string, req DownloadRequest) (*DownloadRespon
 		TrackNumber:      result.TrackNumber,
 		DiscNumber:       result.DiscNumber,
 		ISRC:             result.ISRC,
+		Genre:            req.Genre,
+		Label:            req.Label,
+		Copyright:        req.Copyright,
 	}, nil
 }