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/queries/queries.sql

@@ -1669,6 +1669,14 @@ SET
 WHERE id = $1
 RETURNING *;
 
+-- name: SetDeviceSyncEnabled :one
+UPDATE devices
+SET
+    sync_enabled = $2,
+    updated_at = NOW()
+WHERE id = $1
+RETURNING *;
+
 -- name: UpdateDeviceLastSeen :one
 UPDATE devices
 SET