Fix Edit/Delete on notes saved before properties.id existed

The earliest notes (saved before the previous commit added id
tracking) have no properties.id — update/delete target a feature by
that id, so offering Edit/Delete for one without it just 404s
("Could not delete/save the note", the generic fetch-failure alert).

The popup no longer offers Edit/Delete without an id. New
rake gpx:backfill_note_ids assigns one to any pre-existing Note
feature missing it, so already-saved notes become editable again too
— run once against wherever data/gpx actually lives (e.g. inside the
deployed container).

TODO.adoc

@@ -44,6 +44,7 @@ independence from connectivity.
 ** [x] Save a distance/area shape as a note too — Measure already draws these by hand, so a "Save" button there (not a second click-to-draw UI in the notes tool) reuses that instead of duplicating it; both share one `NoteSaveForm` (notes.js). `gpx.js`'s `_addLayers` now renders Polygon and note-flagged LineString features (fill/outline, dashed line), not just points
 ** [x] Edit/delete individual notes from their popup (logged-in only) — targets a feature by a client-generated `properties.id` (`GpxStore#update_feature`/`#delete_feature`), not array position
 ** [x] Measure's vertices are draggable (mousedown + move, `event.preventDefault()` to stop the map's own drag-pan) and deletable (click to select — shown in yellow — then Delete/Backspace)
+** [x] Fixed: a note saved before `properties.id` existed (the earliest test notes) had no Edit/Delete-able target — the popup no longer offers them without an id, and `rake gpx:backfill_note_ids` assigns one to any pre-existing Note feature missing it
 ** [x] Gated behind login: NotesControl isn't added to the map at all when logged out, Measure's "Save as note" button is hidden (Measure itself stays usable either way)
 ** [ ] The Unigrid Matrix category filter only re-slices the *points* source (`_applyCategoryFilter`) — toggling the "Note" category off hides point notes but not polygon/line 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