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.

app.rb

@@ -2,6 +2,7 @@ require "sinatra/base"
 require "sinatra/json"
 require_relative "lib/maptool/gpx"
 require_relative "lib/maptool/gpx_store"
+require_relative "lib/maptool/gpsd_client"
 
 module Maptool
   class App < Sinatra::Base
@@ -11,6 +12,34 @@ module Maptool
     OFFLINE_PMTILES_PATH = File.join(__dir__, "data", "offline.pmtiles")
     GPX_STORE = GpxStore.new(File.join(__dir__, "data", "gpx"))
 
+    # Live GNSS position, polled by the frontend from /api/gnss/position. A
+    # single background thread reads gpsd (if it's running — harmless retry
+    # loop if not, see GpsdClient) and keeps this updated; polling instead
+    # of push (SSE/WebSocket) is deliberate — gpsd itself only reports at
+    # ~1Hz, so there is nothing to gain from a push channel, and it avoids
+    # Sinatra's `stream(:keep_open)` helper, which — absent an EventMachine
+    # reactor (i.e. under plain threaded Puma, as here) — busy-loops calling
+    # its block repeatedly instead of waiting for external writes; fine for
+    # a single blocking per-message write, not for "register once, get
+    # pushed to later" like this.
+    # A fix is only reported back as "available" for this long after it
+    # last arrived — otherwise a receiver that's lost its fix, or been
+    # unplugged, would keep showing its last position forever as if it
+    # were still live.
+    GNSS_STALE_AFTER = 10 # seconds
+
+    GNSS_LATEST = {}
+    GNSS_LATEST_MUTEX = Mutex.new
+
+    Thread.new do
+      GpsdClient.new.each_position do |position|
+        GNSS_LATEST_MUTEX.synchronize do
+          GNSS_LATEST.replace(position)
+          GNSS_LATEST[:received_at] = Time.now.to_f
+        end
+      end
+    end
+
     get "/" do
       erb :index
     end
@@ -109,5 +138,13 @@ module Maptool
 
       json path: path, tree: GPX_STORE.tree
     end
+
+    get "/api/gnss/position" do
+      GNSS_LATEST_MUTEX.synchronize do
+        received_at = GNSS_LATEST[:received_at]
+        fresh = received_at && (Time.now.to_f - received_at) < GNSS_STALE_AFTER
+        json(fresh ? GNSS_LATEST.merge(available: true) : { available: false })
+      end
+    end
   end
 end