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/icons.js

@@ -0,0 +1,39 @@
+// Small hand-authored monoline SVG icons for the sidebar, so the app
+// doesn't need to vendor an icon font/library for four glyphs.
+
+export const LAYERS_ICON = `
+<svg viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="1.6" stroke-linejoin="round" stroke-linecap="round">
+  <path d="M12 3 2 9l10 6 10-6-10-6z"/>
+  <path d="M2 14l10 6 10-6"/>
+</svg>`;
+
+export const GPX_ICON = `
+<svg viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="1.6" stroke-linecap="round" stroke-linejoin="round">
+  <circle cx="5" cy="6" r="2"/>
+  <circle cx="19" cy="18" r="2"/>
+  <path d="M6.5 7.5C10 11 8 13 12 13s2 5 6.5 5"/>
+</svg>`;
+
+export const RADAR_ICON = `
+<svg viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="1.6" stroke-linejoin="round" stroke-linecap="round">
+  <path d="M7 18a4 4 0 1 1 .7-7.94A5 5 0 0 1 17 9a4 4 0 0 1 0 8H7z"/>
+</svg>`;
+
+export const GNSS_ICON = `
+<svg viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="1.6" stroke-linecap="round">
+  <circle cx="12" cy="12" r="5.5"/>
+  <circle cx="12" cy="12" r="1.4" fill="currentColor" stroke="none"/>
+  <path d="M12 2v3M12 19v3M2 12h3M19 12h3"/>
+</svg>`;
+
+export const NEW_FOLDER_ICON = `
+<svg viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="1.6" stroke-linecap="round" stroke-linejoin="round">
+  <path d="M3 7a2 2 0 0 1 2-2h4l2 2h8a2 2 0 0 1 2 2v8a2 2 0 0 1-2 2H5a2 2 0 0 1-2-2V7z"/>
+  <path d="M12 11v5M9.5 13.5h5"/>
+</svg>`;
+
+export const UPLOAD_ICON = `
+<svg viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="1.6" stroke-linecap="round" stroke-linejoin="round">
+  <path d="M12 15V4M8 8l4-4 4 4"/>
+  <path d="M4 15v3a2 2 0 0 0 2 2h12a2 2 0 0 0 2-2v-3"/>
+</svg>`;