Redesign UI as an OsmAnd-style sidebar, add live GNSS position via gpsd

Restructure the map controls from MapLibre corner buttons into a left
sidebar of icons with flyouts/panels (radar, GPX, GNSS), matching
OsmAnd's layout more closely:

- Base map and GPX selection are now full-height side panels docked
  next to the sidebar (not small dropdown flyouts or a right-docked
  panel), opening/closing exclusively via a small shared event bus
  (panels.js) so only one is ever open at a time.
- The GPX panel's upload/new-folder controls are now icon buttons in
  the header instead of an always-visible dropzone and form; dragging
  a GPX file onto the tree, a specific folder, or straight onto the
  map itself all upload it.
- New: Maptool::GpsdClient (lib/maptool/gpsd_client.rb) streams live
  GNSS fixes from a local gpsd daemon over Server-Sent Events
  (/api/gnss/stream) to a marker on the map (public/js/gnss.js) — one
  background gpsd connection shared by every browser tab.
- The online map style (public/js/osmand-style.js) is now styled to
  resemble OsmAnd's own rendering, applied to both the online and
  offline (PMTiles) styles.

Also adds a GeolocateControl (browser geolocation) next to the zoom
controls.

public/js/panels.js

@@ -0,0 +1,17 @@
+// Keeps at most one full-height side panel (base map, GPX, ...) open at a
+// time — they all dock to the same spot next to the sidebar, so two open
+// together would just stack on top of each other.
+const OPEN_EVENT = "side-panel-open";
+
+export function notifySidePanelOpen(panel) {
+  document.dispatchEvent(new CustomEvent(OPEN_EVENT, { detail: panel }));
+}
+
+export function onlyOneSidePanelOpen(panel, onClosedByOther) {
+  document.addEventListener(OPEN_EVENT, (event) => {
+    if (event.detail !== panel && !panel.hidden) {
+      panel.hidden = true;
+      onClosedByOther();
+    }
+  });
+}