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 —
+load, to default to the offline style when local tile data exists.
-otherwise it defaults to the online style. Either way, the layer
+When it doesn't, `LayerToggleControl#remove()` (`public/js/map.js`)
-toggle in the top-left of the map lets the user switch manually.
+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`

public/js/map.js

@@ -101,6 +101,14 @@ class LayerToggleControl {
       button.classList.toggle("active", button.dataset.style === name);
     });
   }
+
+  // Called once it's known there's no offline map to switch to (see the
+  // /api/offline-tiles check below) — an Online/Offline switch doesn't make
+  // sense to show at all when Offline was never going to work.
+  remove() {
+    this._container.remove();
+    this._panel.hidden = true;
+  }
 }
 
 const map = new MaplibreMap({
@@ -145,12 +153,21 @@ function applyStyle(name) {
 }
 
 // If a local PMTiles file is present, prefer it as the default style — its
-// presence signals the app is meant to be used offline here.
+// presence signals the app is meant to be used offline here. If one isn't,
+// the Online/Offline switch itself doesn't make sense (there's nothing to
+// switch to), so it's removed entirely rather than left sitting there
+// offering a choice that can't work.
 fetch("/api/offline-tiles")
   .then((response) => response.json())
   .then(({ available }) => {
-    if (available) applyStyle("offline");
+    if (available) {
+      applyStyle("offline");
+    } else {
+      layerToggle.remove();
+    }
   })
   .catch(() => {
-    // Backend check failed; keep the online style.
+    // Backend check failed; keep the online style and leave the switch as
+    // is rather than guessing — this is the same-origin request itself
+    // failing, not evidence offline tiles are actually unavailable.
   });