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.

lib/maptool/gpsd_client.rb

@@ -0,0 +1,57 @@
+require "socket"
+require "json"
+
+module Maptool
+  # A minimal client for gpsd's JSON protocol (https://gpsd.io/gpsd_json.html).
+  # Connects over TCP, asks gpsd to watch for reports, and yields a plain
+  # position hash for every TPV ("time-position-velocity") report — gpsd's
+  # term for a GNSS fix — for as long as the block keeps running.
+  class GpsdClient
+    RECONNECT_DELAY = 5 # seconds, after gpsd is unreachable or drops us
+
+    def initialize(host: "127.0.0.1", port: 2947)
+      @host = host
+      @port = port
+    end
+
+    # Never returns under normal operation: reconnects on any I/O error
+    # (gpsd not running yet, connection dropped, etc.) after a short delay,
+    # so a missing/disconnected receiver is not fatal to the caller.
+    def each_position
+      loop do
+        watch { |position| yield position }
+      rescue SystemCallError, EOFError, SocketError => e
+        warn "[gpsd] #{e.class}: #{e.message} — retrying in #{RECONNECT_DELAY}s"
+        sleep RECONNECT_DELAY
+      end
+    end
+
+    private
+
+    def watch
+      socket = TCPSocket.new(@host, @port)
+      socket.puts('?WATCH={"enable":true,"json":true};')
+
+      socket.each_line do |line|
+        report = JSON.parse(line)
+        next unless report["class"] == "TPV"
+        next unless report["lat"] && report["lon"]
+
+        yield(
+          lat: report["lat"],
+          lon: report["lon"],
+          # altHAE (height above ellipsoid) is gpsd's modern field; older
+          # gpsd versions only send "alt".
+          alt: report["altHAE"] || report["alt"],
+          speed: report["speed"],
+          track: report["track"],
+          mode: report["mode"], # 0/1 = no fix, 2 = 2D, 3 = 3D
+          eph: report["eph"], # estimated horizontal position error, metres
+          time: report["time"],
+        )
+      end
+    ensure
+      socket&.close
+    end
+  end
+end