Hide the Base map switch when there's no offline map to switch to

The Online/Offline sidebar icon was always shown, even on an install with
no data/offline.pmtiles — offering a choice that can't actually work.
LayerToggleControl#remove() removes the icon (and hides its panel) once the
existing /api/offline-tiles check confirms there's nothing to switch to; a
transient failure of that same-origin check leaves the switch as is rather
than guessing, since that's the request itself failing, not evidence
offline tiles are unavailable.

docs/architecture.adoc

@@ -59,9 +59,11 @@ view (`public/js/map.js`) offers two styles:
 
 `GET /api/offline-tiles` reports whether `data/offline.pmtiles` is
 present (`{"available": true|false}`); the frontend uses it once, on
-load, to default to the offline style when local tile data exists —
-otherwise it defaults to the online style. Either way, the layer
-toggle in the top-left of the map lets the user switch manually.
+load, to default to the offline style when local tile data exists.
+When it doesn't, `LayerToggleControl#remove()` (`public/js/map.js`)
+removes the Online/Offline sidebar icon entirely — a switch offering a
+choice that can't work is worse than no switch at all — so it's only
+ever shown on an install that actually has a PMTiles file to switch to.
 
 PMTiles is a single, self-indexed file, which is why this needs no
 tile-decoding logic on the server: `app.rb`'s `/tiles/offline.pmtiles`