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.

docs/development.adoc

@@ -42,3 +42,51 @@ build one locally:
 
 Once the file is in place, `GET /api/offline-tiles` reports
 `available: true` and the map defaults to the offline style.
+
+== GNSS position (gpsd)
+
+The "GNSS" control needs https://gpsd.io[gpsd] running and pointed at
+a receiver (`gpsd /dev/ttyUSB0`, or whatever device the receiver shows
+up as). On openSUSE, `gpsd` is a package with systemd units
+(`gpsd.service` / `gpsd.socket`); on other distributions, check your
+package manager. No app-side configuration is needed — it just
+connects to `127.0.0.1:2947`, gpsd's default port.
+
+To test without hardware, feed gpsd a fake NMEA stream over TCP
+instead of a real serial device — gpsd accepts a `tcp://host:port`
+source:
+
+[source,shell]
+----
+# terminal 1: a TCP server repeating one NMEA GGA/RMC sentence pair
+python3 - <<'PY' 5001
+import socket, sys, time
+
+def checksum(s):
+    cs = 0
+    for ch in s:
+        cs ^= ord(ch)
+    return f"{cs:02X}"
+
+def sentence(body):
+    return f"${body}*{checksum(body)}\r\n"
+
+port = int(sys.argv[1])
+srv = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
+srv.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
+srv.bind(("127.0.0.1", port))
+srv.listen(1)
+conn, _ = srv.accept()
+gga = "GPGGA,123519,4808.916,N,01706.462,E,1,08,0.9,140.4,M,46.9,M,,"
+rmc = "GPRMC,123519,A,4808.916,N,01706.462,E,000.5,084.4,230824,003.1,W"
+while True:
+    conn.sendall((sentence(gga) + sentence(rmc)).encode())
+    time.sleep(1)
+PY
+
+# terminal 2: point gpsd at it instead of a real device
+gpsd -N -n tcp://127.0.0.1:5001
+----
+
+`GET /api/gnss/position` should then report the fake fix (Bratislava
+Castle, in the sentences above) within a couple of seconds.