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.

public/css/style.css

@@ -740,6 +740,96 @@ html, body {
   color: #333;
 }
 
+.notes-ctrl {
+  position: relative;
+}
+
+.notes-ctrl__button svg {
+  display: block;
+  margin: 0 auto;
+  width: 18px;
+  height: 18px;
+}
+
+.notes-ctrl__button.active {
+  background: #dbe9ff;
+}
+
+.notes-panel {
+  position: absolute;
+  top: 0;
+  left: 100%;
+  margin-left: 8px;
+  z-index: 20;
+  width: 200px;
+  background: #fff;
+  border: 1px solid #ddd;
+  border-radius: 6px;
+  box-shadow: 0 2px 8px rgba(0, 0, 0, 0.15);
+  padding: 8px;
+  font-size: 13px;
+}
+
+.notes-panel[hidden] {
+  display: none;
+}
+
+.notes-panel__hint {
+  margin: 0;
+  color: #888;
+}
+
+.notes-panel__form {
+  display: flex;
+  flex-direction: column;
+  gap: 6px;
+}
+
+.notes-panel__text {
+  width: 100%;
+  box-sizing: border-box;
+  font: inherit;
+  font-size: 12px;
+  padding: 4px 6px;
+  border: 1px solid #ddd;
+  border-radius: 4px;
+  resize: vertical;
+}
+
+.notes-panel__target,
+.notes-panel__new-name {
+  width: 100%;
+  box-sizing: border-box;
+  font: inherit;
+  font-size: 12px;
+  padding: 4px 6px;
+  border: 1px solid #ddd;
+  border-radius: 4px;
+}
+
+.notes-panel__actions {
+  display: flex;
+  justify-content: flex-end;
+  gap: 6px;
+}
+
+.notes-panel__actions button {
+  width: auto;
+  height: auto;
+  padding: 4px 10px;
+  border: none;
+  border-radius: 4px;
+  background: #f0f0f0;
+  font: inherit;
+  font-size: 12px;
+  cursor: pointer;
+}
+
+.notes-panel__save {
+  background: #1a5fb4;
+  color: #fff;
+}
+
 .zoom-level-ctrl__value {
   width: 29px;
   height: 29px;