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.

views/index.erb

@@ -1,21 +1,43 @@
-<div id="map"></div>
+<div id="app">
+  <header class="topbar">
+    <span class="topbar__brand">MapTool</span>
+  </header>
+
+  <div class="app-body">
+    <nav id="sidebar" class="sidebar"></nav>
+    <div id="map"></div>
+  </div>
+</div>
+
+<div id="basemap-panel" class="basemap-panel" hidden>
+  <div class="basemap-panel__header">
+    <strong>Base map</strong>
+    <button type="button" class="basemap-panel__close" aria-label="Close">&times;</button>
+  </div>
+  <div class="basemap-panel__list">
+    <button type="button" class="basemap-panel__option" data-style="online">
+      <span>Online</span>
+      <span class="basemap-panel__check">&check;</span>
+    </button>
+    <button type="button" class="basemap-panel__option" data-style="offline">
+      <span>Offline</span>
+      <span class="basemap-panel__check">&check;</span>
+    </button>
+  </div>
+</div>
 
 <div id="gpx-panel" class="gpx-panel" hidden>
   <div class="gpx-panel__header">
     <strong>GPX</strong>
-    <button type="button" class="gpx-panel__close" aria-label="Close">&times;</button>
+    <div class="gpx-panel__actions">
+      <button type="button" class="gpx-panel__action" data-action="new-folder" title="New folder" aria-label="New folder"></button>
+      <button type="button" class="gpx-panel__action" data-action="upload" title="Upload GPX file" aria-label="Upload GPX file"></button>
+      <button type="button" class="gpx-panel__close" aria-label="Close">&times;</button>
+    </div>
   </div>
 
-  <div class="gpx-dropzone">
-    Drop .gpx files here, or click to browse
-    <input type="file" class="gpx-dropzone__input" accept=".gpx" multiple hidden>
-  </div>
+  <input type="file" class="gpx-file-input" accept=".gpx" multiple hidden>
   <p class="gpx-upload__status" hidden></p>
 
-  <form class="gpx-new-folder">
-    <input type="text" name="path" placeholder="new folder path (e.g. Hiking/2026)">
-    <button type="submit">+ folder</button>
-  </form>
-
   <div class="gpx-tree"></div>
 </div>