Add live contact search to invitee input

The static datalist only carried the first 50 address book
contacts, so typed fragments could not match anything beyond
that. A new read-only GET /contacts/search endpoint queries
the full address book per keystroke, and a small vanilla JS
combobox shows the results and replaces only the fragment
after the last separator, so comma-separated lists keep
working. The server-rendered datalist remains as the no-JS
fallback; the JS removes the list attribute at runtime so
the native popup does not double the custom one.

appinfo/routes.php

@@ -14,5 +14,10 @@ return [
             'url' => '/create',
             'verb' => 'POST',
         ],
+        [
+            'name' => 'calendar#searchContacts',
+            'url' => '/contacts/search',
+            'verb' => 'GET',
+        ],
     ],
 ];