Log an event that happened "now" in one tap

Logging a quick pee/poo/meal/sleep boundary took two taps: open the note
dialog, then Save. Make a single tap on a quick-action log the event
immediately at the current time.

Non-weight quick actions now call addEvent(type, "", Date.now()) directly
and show a brief snackbar with Undo (deletes the just-added event) and Add
note (opens the edit dialog for it), so notes, photos and custom times are
never lost. Weigh-ins still open the dialog since they need a value.
addEvent now returns the created event so the snackbar can reference it.

src/index.html

@@ -303,6 +303,13 @@
       <img id="lightbox-img" alt="" />
     </dialog>
 
+    <!-- Brief confirmation after a one-tap quick log, with Undo / Add note. -->
+    <div id="snackbar" class="snackbar" hidden role="status" aria-live="polite">
+      <span id="snackbar-msg" class="snackbar-msg"></span>
+      <button type="button" id="snackbar-note" class="snackbar-action">Add note</button>
+      <button type="button" id="snackbar-undo" class="snackbar-action">Undo</button>
+    </div>
+
     <script src="app.js"></script>
   </body>
 </html>