Consolidate note-drawing into Measure; add edit/delete

Measure already draws distance lines and area polygons by hand — a
second click-to-draw UI in the notes tool (Point/Area mode, from the
previous commit) duplicated that. Replaced it with a "Save as note"
button on the Measure panel that saves whatever shape is currently
drawn there; NotesControl goes back to being just a one-click point
pin. Both share a single NoteSaveForm (notes.js) for the note-text +
target-layer-file UI instead of two copies of it.

gpx.js: the "lines" source now also renders a note-flagged LineString
(dashed, popup-on-click) alongside the Polygon fill/outline from
before — the plain track-line layer excludes category:"Note" features
so a saved distance note doesn't also get the track hover-summary/
profile-panel treatment.

Notes are now individually editable/deletable from their own popup
(logged-in only): Edit swaps the popup content for an inline
textarea, Delete confirms and removes. Both target the feature by a
new client-generated properties.id rather than array position (new
GpxStore#update_feature/#delete_feature, PUT/DELETE /api/gpx/notes),
since neither array index nor the note's own text (the thing being
edited) is a stable identifier.

Verified live: Measure's Save button (disabled/enabled state, saves
the exact on-screen shape), the popup's Edit/Delete buttons and inline
edit form. Full test suite: 115 runs, 0 failures (new coverage for
update_feature/delete_feature and the PUT/DELETE routes).

public/css/style.css

@@ -673,9 +673,6 @@ html, body {
   left: 100%;
   margin-left: 8px;
   z-index: 20;
-  display: flex;
-  align-items: center;
-  gap: 6px;
   min-width: max-content;
   background: #fff;
   border: 1px solid #ddd;
@@ -689,6 +686,41 @@ html, body {
   display: none;
 }
 
+.measure-panel__controls {
+  display: flex;
+  align-items: center;
+  gap: 6px;
+}
+
+/* Same "author display wins over the UA [hidden] rule" gotcha as
+   .gpx-panel__action[hidden]/.notes-panel__form[hidden] elsewhere in this
+   file — this sets its own "display" too. */
+.measure-panel__controls[hidden] {
+  display: none;
+}
+
+.measure-panel .measure-panel__save {
+  width: auto;
+  height: auto;
+  display: inline-block;
+  border: none;
+  border-radius: 4px;
+  background: none;
+  font-size: 14px;
+  line-height: 1;
+  cursor: pointer;
+  padding: 2px 4px;
+}
+
+.measure-panel__save:disabled {
+  opacity: 0.3;
+  cursor: default;
+}
+
+.measure-panel .notes-panel__form {
+  width: 200px;
+}
+
 /* ".measure-panel " prefix (not just ".measure-panel__mode") to outrank
    ".maplibregl-ctrl-group button" (higher specificity, since these buttons
    sit inside a maplibregl-ctrl-group container for the ruler icon's own
@@ -990,6 +1022,42 @@ html, body {
   font-size: 12px;
 }
 
+.gpx-popup__note-actions,
+.gpx-popup__edit-actions {
+  display: flex;
+  gap: 6px;
+  margin-top: 8px;
+  padding-top: 6px;
+  border-top: 1px solid #eee;
+}
+
+.gpx-popup__note-actions button,
+.gpx-popup__edit-actions button {
+  border: none;
+  border-radius: 4px;
+  background: #f0f0f0;
+  padding: 3px 8px;
+  font: inherit;
+  font-size: 12px;
+  cursor: pointer;
+}
+
+.gpx-popup__edit-actions .gpx-popup__edit-save {
+  background: #1a5fb4;
+  color: #fff;
+}
+
+.gpx-popup__edit-text {
+  width: 100%;
+  box-sizing: border-box;
+  font: inherit;
+  font-size: 13px;
+  padding: 4px 6px;
+  border: 1px solid #ddd;
+  border-radius: 4px;
+  resize: vertical;
+}
+
 .gnss-control__status {
   color: #888;
   white-space: nowrap;