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).

views/index.erb

@@ -2,6 +2,7 @@
   <header class="topbar">
     <span class="topbar__brand" title="rev <%= revision %>">MapTool</span>
     <span class="topbar__coords"></span>
+    <span class="topbar__offline-status" hidden></span>
     <div class="topbar__auth">
       <% if logged_in %>
         <span><%= user %></span>