Fix stale contact suggestions reopening

hide() did not invalidate in-flight search requests. Clearing
the input or picking a contact while a request was pending let
the old response re-render and pop the suggestion list back
open. Bump requestSeq in hide() so every dismissal retires
pending responses.

Add a Vitest + jsdom harness with black-box tests for the
invitee widget, covering the stale-response race both on
field clear and on contact pick.

js/invitees.js

@@ -49,6 +49,7 @@
   }
 
   function hide() {
+    requestSeq++;
     list.hidden = true;
     list.replaceChildren();
     suggestions = [];