Add a "user overlay" tool: click the map, save a note

New map-corner "Add note" control (public/js/notes.js), mirroring
measure.js's click-to-place UX. Click the map, write a short note,
pick an existing .geojson layer file or name a new one — saved via a
new GpxStore#append_feature (grows the target file in place, unlike
save()'s always-disambiguate-the-filename behavior) through a new
POST /api/gpx/notes route.

No new rendering path: a note is just another point in a .geojson
file the Tracks panel's tree already lists, checkboxes, and pops up —
visibility toggling ("layers") comes for free from that existing
machinery, per-file, with no changes needed there.

Points only for now — a polygon mode would need gpx.js's point/line
split taught a third Polygon bucket first, or a saved polygon
wouldn't render back on reload (see TODO.adoc).

Covered by new tests: GpxStore#append_feature (create/append/missing-
path/filename-safety) and the /api/gpx/notes route (auth gate, create,
append, malformed/missing-geometry rejection, 404 on a missing
target). Full suite: 97 runs, 0 failures.

TODO.adoc

@@ -40,7 +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)
-* [ ] User overlay: offline drawing of custom points/polygons with notes (e.g. "turning space for a long vehicle", "narrow gate", "good swimming spot")
+* [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
 * [ ] 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)