fix(sync): offline detector wiped device name, auto_sync, and metadata on every transition

setOfflineMode/setOnlineMode only mean to flip sync_enabled, but they
called the full-row UpdateDevice with Go zero values — which wrote
DeviceName: "" and NULLed auto_sync/sync_frequency_minutes/
device_metadata on the device row at every offline->online flip. New
targeted SetDeviceSyncEnabled query touches exactly that column; the
real settings update keeps using UpdateDevice.

Caught as part of the H4 helper-census pass (plan §5.4 erasure list).

internal/database/querier.go

@@ -526,6 +526,7 @@ type Querier interface {
 	SearchNarratorValues(ctx context.Context, arg SearchNarratorValuesParams) ([]SearchNarratorValuesRow, error)
 	SearchSeriesValues(ctx context.Context, arg SearchSeriesValuesParams) ([]SearchSeriesValuesRow, error)
 	SearchTagsValues(ctx context.Context, arg SearchTagsValuesParams) ([]SearchTagsValuesRow, error)
+	SetDeviceSyncEnabled(ctx context.Context, arg SetDeviceSyncEnabledParams) (Devices, error)
 	// Library Visibility queries
 	SetLibraryVisibility(ctx context.Context, arg SetLibraryVisibilityParams) (LibraryVisibility, error)
 	// Set system config