Start the NPS Unigrid visual redesign

Black topbar banner with live map-center coordinates, a black header
(with NPS pictogram) on waypoint popups, offline basemap wood/park
colors unified with the online Liberty green, and the sidebar grouped
into the planned Explorer / Offline hub / Navigation menu sections.

views/index.erb

@@ -1,6 +1,7 @@
 <div id="app">
   <header class="topbar">
     <span class="topbar__brand" title="rev <%= revision %>">MapTool</span>
+    <span class="topbar__coords"></span>
     <div class="topbar__auth">
       <% if logged_in %>
         <span><%= user %></span>