Add an offline-data status dot to the topbar

Small green/gray indicator reusing the existing /api/offline-tiles
check, showing at a glance whether this server has offline map data.
Closes out the top banner TODO item — DEM elevation and a hamburger
menu are intentionally left out (no DEM layer yet, and the sidebar
isn't mobile-collapsed).

public/js/map.js

@@ -442,6 +442,7 @@ function applyStyle(name) {
 // 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.
+const offlineStatus = document.querySelector(".topbar__offline-status");
 fetch("/api/offline-tiles")
   .then((response) => response.json())
   .then(({ available }) => {
@@ -451,6 +452,9 @@ fetch("/api/offline-tiles")
       layerToggle.remove();
       map.removeControl(mapLayersButton);
     }
+    offlineStatus.hidden = false;
+    offlineStatus.classList.toggle("topbar__offline-status--ready", available);
+    offlineStatus.title = available ? "Offline map data ready" : "No offline map data on this server";
   })
   .catch(() => {
     // Backend check failed; keep the online style and leave the switch as