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.

templates/confirm.php

@@ -141,6 +141,7 @@ $l = $_['l10n'];
                             name="attendees"
                             type="text"
                             list="atocal-contact-emails"
+                            data-search-url="<?= $h($_['contactsSearchUrl']) ?>"
                             value="<?= $h($_['attendees']) ?>"
                             placeholder="<?= $h(
                                 $l->t('Email addresses, comma-separated')