An install that never went online while visiting /maps never provisions
world.pmtiles (it is lazily extracted from the internet), leaving the map
blank/grey offline outside any downloaded regional extracts.
Detect the world basemap's presence on disk and surface it to the UI so:
- /maps shows a clear notice explaining the base map isn't downloaded and
how to fix it, instead of a silent grey screen.
- /settings/maps offers an explicit "Download Base Map" action (~15 MB) to
provision it deliberately while online.
Adds MapService.checkWorldBasemapExists() / provisionWorldBasemap(), a
POST /maps/setup-world-basemap endpoint, and threads worldBasemapExists
through the maps page props.
Refs #1030
Co-authored-by: Cursor <cursoragent@cursor.com>
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