loadOnlineBaseLayer

Load the online basemap fallback for properties that have no per-property MBTiles (mapTilesFilerId null/empty) and no global baseMapTiles.filerId. This is the ONLY base layer shown in that scenario — Carto and the Mapsted MBTiles layer are never stacked together.

MUST be called on the UI thread. Idempotent — safe to call multiple times. Does NOT modify mBaseLayer / mVectorTileDataSource so the legacy MBTiles refresh path remains intact.