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/css/style.css

@@ -34,6 +34,18 @@ html, body {
   color: #bbb;
 }
 
+.topbar__offline-status {
+  width: 8px;
+  height: 8px;
+  border-radius: 50%;
+  background: #666;
+  flex-shrink: 0;
+}
+
+.topbar__offline-status--ready {
+  background: #2a9d8f;
+}
+
 .topbar__auth {
   margin-left: auto;
   display: flex;