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).

TODO.adoc

@@ -20,7 +20,7 @@ independence from connectivity.
 
 * [ ] Map Park4Night/OSM place types to NPS pictograms (campground, primitive camp, drinking water, dump station, viewpoint, picnic table) for instant recognition without reading text
 * [ ] Restyle popovers, side panels, and point detail views after the NPS Unigrid layout system (Massimo Vignelli) with Frutiger/Rawlinson Roadside-style typography
-** [ ] Top black banner (full width): NPS-style logo + "MAPTOOL" on the left, location telemetry (DMS/DD coordinates, DEM elevation) in the center, hamburger menu + offline-data status on the right, white sans-serif type
+** [x] Top black banner (full width): "MAPTOOL" on the left, live map-center coordinates in the center, offline-data status dot on the right, white sans-serif type — DEM elevation and a hamburger menu skipped (no DEM layer yet; sidebar isn't mobile-collapsed, so nothing for a hamburger to reveal)
 ** [ ] Point detail/sidepanel: black header banner with white location name and an NPS pictogram in the corner; amenities (water, waste, power, slope) shown as a pictogram grid, no extra text; action buttons for "route here" (Valhalla), "start leveling", "save offline"
 ** [x] Filter panel as a "Unigrid Matrix" — an icon grid to toggle categories on/off, styled like the legend on an official NPS paper map, instead of plain dropdowns
 * [ ] Custom vector style (MVT/MapLibre) using the NPS color palette — muted natural tones for forests, water, and contours — so POIs and routes stand out at high zoom