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/sync/offline.go

@@ -133,11 +133,12 @@ func (d *OfflineDetector) handleReconnectedDevice(ctx context.Context, status De
 }
 
 func (d *OfflineDetector) setOfflineMode(ctx context.Context, deviceID pgtype.UUID) {
-	_, err := d.db.UpdateDevice(ctx, database.UpdateDeviceParams{
+	// Targeted flip: the full-row UpdateDevice used here zeroed the
+	// device name, auto_sync, frequency, and metadata on every
+	// transition (pg zero values are not "leave alone").
+	_, err := d.db.SetDeviceSyncEnabled(ctx, database.SetDeviceSyncEnabledParams{
 		ID:          deviceID,
-		DeviceName:  "",
 		SyncEnabled: pgtype.Bool{Bool: false, Valid: true},
-		AutoSync:    pgtype.Bool{},
 	})
 
 	if err != nil {
@@ -146,11 +147,12 @@ func (d *OfflineDetector) setOfflineMode(ctx context.Context, deviceID pgtype.UU
 }
 
 func (d *OfflineDetector) setOnlineMode(ctx context.Context, deviceID pgtype.UUID) {
-	_, err := d.db.UpdateDevice(ctx, database.UpdateDeviceParams{
+	// Targeted flip: the full-row UpdateDevice used here zeroed the
+	// device name, auto_sync, frequency, and metadata on every
+	// transition (pg zero values are not "leave alone").
+	_, err := d.db.SetDeviceSyncEnabled(ctx, database.SetDeviceSyncEnabledParams{
 		ID:          deviceID,
-		DeviceName:  "",
 		SyncEnabled: pgtype.Bool{Bool: true, Valid: true},
-		AutoSync:    pgtype.Bool{},
 	})
 
 	if err != nil {