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/architecture.adoc

@@ -83,6 +83,46 @@ No PMTiles file ships with the repository — it is expected to be
 supplied locally (see `docs/development.adoc`) and stays gitignored
 because such extracts are typically large binary files.
 
+=== Visual style
+
+Both styles are recolored, by `public/js/osmand-style.js`, to
+approximate the look of https://osmand.net/map[OsmAnd's own web map]
+(cream background, green forests, orange roads, purple admin
+boundaries) — deliberately *not* by using OsmAnd's tile server
+directly: its terms restrict tile use to "personal, non-commercial use
+within OsmAnd applications and platforms", which this app isn't.
+
+One shared color palette drives two independent recolorings:
+
+* *Online*: `onlineStyle()` fetches OpenFreeMap's raw "liberty" style
+  JSON and patches `paint` colors on its layers in place — matching a
+  layer to a category (water, wood, park, landuse, building, major/
+  minor road, boundary) by its id, then walking that layer's color
+  properties (`background-color`, `fill-color`, `line-color`,
+  `fill-extrusion-color`, and any `*-outline-color`) and replacing
+  every color-shaped leaf — including inside `interpolate`/other
+  expressions — with the category's palette color, leaving non-color
+  leaves (operators, zoom stops, `["get", ...]`) untouched. Labels and
+  railways are left at their original colors — only categorized layer
+  types (`background`/`fill`/`line`/`fill-extrusion`) are touched at
+  all, so `symbol` layers (all labels/icons) are never in scope.
+  Result is cached after the first fetch.
+* *Offline*: `offlineTheme()` overrides the relevant color roles on a
+  `protomaps-themes-base` `Theme` object (starting from
+  `namedTheme("light")`), leaving roles it doesn't set (labels — moot,
+  see above — rail, tunnels, POI colors) at their defaults.
+
+=== Known limitation: coarser road colors offline
+
+protomaps-themes-base's `Theme` only has two paint-relevant road tiers
+(`major` / `minor_a`+`minor_b`, plus `link` for ramps) versus
+OpenFreeMap's finer-grained classes (`road_motorway`,
+`road_trunk_primary`, `road_secondary_tertiary`, `road_minor`, ...).
+Mapping `major` to the palette's road-major orange colors more roads
+orange offline than online (protomaps' "major" bucket is broader than
+just motorway/trunk/primary) — a real difference in how coarse the two
+underlying style systems are, not a bug in the recoloring itself.
+
 == Weather radar overlay
 
 `public/js/radar.js` adds a "Radar" map control that overlays recent
@@ -177,6 +217,46 @@ assembling any HTML; the URL pattern excludes quote characters, so a
 matched URL can never break out of the `href="..."` attribute. Every
 other value is HTML-escaped before being handed to `Popup#setHTML`.
 
+== Live GNSS position
+
+A "GNSS" map control (`public/js/gnss.js`) shows the fix from a local
+GNSS receiver (a real one, plugged into the machine running the
+server — distinct from `NavigationControl`'s browser
+`navigator.geolocation`, which the OS may or may not derive from such
+a receiver at all).
+
+`lib/maptool/gpsd_client.rb` is a small client for
+https://gpsd.io[gpsd]'s JSON protocol over TCP (port 2947 by default):
+it watches for TPV — "time-position-velocity", gpsd's term for a fix —
+reports and yields each as a plain hash, reconnecting on any I/O error
+so a receiver that isn't plugged in yet (or gpsd not running) is not
+fatal. `app.rb` runs one of these in a background thread for the
+whole process and keeps the latest fix in `GNSS_LATEST`, guarded by a
+mutex since the HTTP threads read it concurrently.
+
+`GET /api/gnss/position`, polled by the frontend every 1.5s, is a
+deliberate choice over a push channel (SSE/WebSocket): gpsd itself
+only reports at roughly 1Hz, so a push channel buys nothing here, and
+it sidesteps a real problem — Sinatra's `stream(:keep_open)` helper,
+absent an EventMachine reactor (i.e. under plain threaded Puma, as
+here), calls its block in a tight loop instead of waiting for external
+writes. That is fine for a stream that itself blocks between writes,
+but for a "register once, get pushed to later" connection — which is
+what a live position feed is — the block returns immediately and gets
+re-invoked immediately, forever: every iteration re-registers the
+connection, so held connections and memory grow unbounded. This was
+caught in testing (the dev server hit ~98% CPU and ~28GB RSS within
+minutes) before it shipped.
+
+The endpoint reports `available: false` once the last fix is more than
+`GNSS_STALE_AFTER` (10s) old, rather than serving a stale position
+forever after the receiver loses its fix or gets unplugged.
+
+MapLibre `Marker`s (used for the fix) are plain positioned DOM
+elements, not tied to the style's sources/layers, so — unlike the
+radar/GPX layers — the marker survives `map.setStyle()` (switching
+Online/Offline) with no extra re-add handling needed.
+
 == Networking
 
 The Sinatra server only binds to `127.0.0.1`. It is not meant to be

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.