Split notes into a title + body, with light self-authored markdown

Both NoteSaveForm (create) and the popup's inline Edit form now have
a title input (-> properties.name, shown in the black header, same as
every other waypoint popup) and a separate body textarea (->
properties.description) for more detail than a single line.

New formatNoteBody renders that body with a small self-rolled
markdown subset — **bold**, *italic*, auto-linked URLs (reusing the
existing linkify), newlines to <br> — deliberately a new function, not
a change to formatDescription: that one renders uploaded GPX/park4night
content, which is untrusted, and docs/nps-resources.adoc already has a
specific call not to add markdown rendering to that path. A note's
body is written by the logged-in viewer themselves, not uploaded, so
that concern doesn't apply here — the escape-then-selectively-markup
order (linkify first, since it does its own escaping, then wrap
already-escaped text in real <strong>/<em> tags) keeps it safe either
way.

Verified live: title renders in the popup header, **bold**/*italic*/
a URL in the body all render correctly. Full test suite: 115 runs, 0
failures (unaffected — description is just another property, not
something the backend routes inspect).

TODO.adoc

@@ -45,6 +45,7 @@ independence from connectivity.
 ** [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] Title + body split (name in the black header, body in a separate field) with light self-authored markdown (`**bold**`, `*italic*`, auto-linked URLs) in the body — a *new*, separate `formatNoteBody` (not `formatDescription`, which stays plain/untrusted-safe for uploaded GPX/park4night content per docs/nps-resources.adoc's earlier markdown-rendering call)
 ** [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