Add polygon notes (Area mode) to the user overlay tool

notes.js: Point/Area mode toggle mirroring measure.js's UX — Area
mode accumulates clicked vertices with a live dashed-preview polygon
(own small preview source/layers), a Finish button once 3+ corners
are placed, then the same save form as Point mode.

gpx.js's _addLayers now also renders Polygon features from a file's
"lines" source (fill + outline, filtered by geometry-type — the
existing line layer already only draws LineString, so this is just
two more layers on the same source, not a new bucket) with
popup-on-click like a waypoint, not the track line's hover-summary/
profile-panel behavior. Without this a saved polygon wouldn't have
rendered back on reload.

Fixed along the way: .notes-panel__form/.notes-panel__modes both set
their own `display`, which silently wins over the browser's default
`[hidden] { display: none }` (author styles always beat user-agent
ones) — same gotcha .gpx-panel__action[hidden] already has a comment
about. The form was never actually invisible before being opened.

Verified live: real polygon file (fill/outline render, click ->
popup) and the full Area-mode draw flow (mode switch, live preview,
Finish, form) via headless Chrome. Full test suite: 104 runs, 0
failures — one new backend test confirms a Polygon feature round-
trips through POST /api/gpx/notes.

Known gap (noted in TODO.adoc): the category filter matrix only
re-slices the points source, so toggling "Note" off hides point notes
but not polygon ones.

TODO.adoc

@@ -40,8 +40,8 @@ independence from connectivity.
 == Custom layers & data
 
 * [x] GPX/GeoJSON drag-and-drop import (Wikiloc, Komoot, own GPX tracks — also FIT), auto-styled by category with a generated elevation profile — KML not supported (only GPX/FIT/GeoJSON/.zip)
-* [x] User overlay: offline drawing of custom points with notes (e.g. "turning space for a long vehicle", "narrow gate", "good swimming spot") — click the map, pick or name a `.geojson` layer, saved into the same GpxStore the Tracks panel already reads (so visibility/popup come for free)
-** [ ] Polygon notes (not just points) — needs gpx.js's point/line split (`_addLayers`) taught a third, Polygon-shaped bucket first, or a saved polygon wouldn't render back on reload
+* [x] User overlay: offline drawing of custom points *and areas* with notes (e.g. "turning space for a long vehicle", "narrow gate", "good swimming spot") — Point/Area mode toggle with a live drawing preview, saved into the same GpxStore the Tracks panel already reads (so visibility/popup come for free); gpx.js's `_addLayers` now renders Polygon features (fill + outline) alongside points/lines, not just points
+** [ ] The Unigrid Matrix category filter only re-slices the *points* source (`_applyCategoryFilter`) — toggling the "Note" category off hides point notes but not polygon ones sharing that category
 * [ ] Offline satellite tile caching (e.g. ESRI) for a selected route area — key for judging surface quality and obstacles when wild camping
 
 == Nice to have (later)