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.

vitest.config.mjs

@@ -0,0 +1,8 @@
+import { defineConfig } from 'vitest/config';
+
+export default defineConfig({
+  test: {
+    environment: 'jsdom',
+    include: ['tests/js/**/*.test.mjs'],
+  },
+});