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).

lib/maptool/gpx_store.rb

@@ -154,6 +154,38 @@ module Maptool
       path
     end
 
+    # Replaces the feature whose properties["id"] matches `id` (see
+    # append_feature/notes.js — every hand-drawn note gets a client-
+    # generated id specifically so it can be found again later, since nothing
+    # else here is a stable identifier: array position shifts as other
+    # features are added/removed, and a note's own text is exactly the thing
+    # being edited). Raises Errno::ENOENT for a missing file *or* a missing
+    # feature — same "nothing to act on" story either way.
+    def update_feature(path:, id:, feature:)
+      file = resolve_file(path)
+      raise Errno::ENOENT, file unless File.file?(file)
+
+      collection = JSON.parse(File.read(file))
+      index = (collection["features"] || []).find_index { |f| f.dig("properties", "id") == id }
+      raise Errno::ENOENT, "#{path}##{id}" unless index
+
+      collection["features"][index] = feature
+      File.write(file, JSON.generate(collection))
+    end
+
+    # Same id-matching story as update_feature above.
+    def delete_feature(path:, id:)
+      file = resolve_file(path)
+      raise Errno::ENOENT, file unless File.file?(file)
+
+      collection = JSON.parse(File.read(file))
+      before = (collection["features"] || []).length
+      collection["features"] = (collection["features"] || []).reject { |f| f.dig("properties", "id") == id }
+      raise Errno::ENOENT, "#{path}##{id}" if collection["features"].length == before
+
+      File.write(file, JSON.generate(collection))
+    end
+
     private
 
     def resolve_dir(folder)