Gate note-writing UI behind login

Adding a note (either tool) always ended in a 401 for a logged-out
viewer — the write-only affordance just wasn't hidden yet, unlike
GpxPanelControl's own upload/new-folder buttons, which already are.

NotesControl isn't added to the map at all when logged out (its whole
purpose is writing); MeasureControl's "Save as note" button is hidden
but the tool itself stays usable for plain measuring either way.

public/css/style.css

@@ -717,6 +717,12 @@ html, body {
   cursor: default;
 }
 
+/* Same "author display wins over the UA [hidden] rule" gotcha as
+   .gpx-panel__action[hidden] elsewhere in this file. */
+.measure-panel__save[hidden] {
+  display: none;
+}
+
 .measure-panel .notes-panel__form {
   width: 200px;
 }