Make Measure's vertices draggable and deletable

Drag: mousedown on a vertex circle calls event.preventDefault() (the
documented way to opt a gesture out of MapLibre's own drag-to-pan),
then a map-wide mousemove/mouseup follows the cursor and updates that
point's coordinates live. Delete: clicking a vertex selects it (shown
larger/yellow); Delete or Backspace removes it, guarded against
eating a keystroke meant for an actual text input on the page.

A vertex's own mousedown sets a one-shot flag so the same click
doesn't also fall through to the generic map-click handler and add a
brand new point on top of it.

TODO.adoc

@@ -43,6 +43,8 @@ independence from connectivity.
 * [x] User overlay: offline drawing of custom points with notes (e.g. "turning space for a long vehicle", "narrow gate", "good swimming spot") — one click on the map, pick or name a `.geojson` layer, saved into the same GpxStore the Tracks panel already reads (so visibility/popup come for free)
 ** [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] 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