Replace as-you-type account check with an explicit search field

The kiosk previously ran a debounced background lookup on every keystroke
in the visitor Email/Phone fields, silently autofilling matches. Replaced
with a dedicated "Look up existing account" field that only searches when
the operator presses Enter or clicks Search, and matches exactly against
either email or phone (never both from one query, to avoid digits in an
email being misread as an unrelated phone number). Also simplified the
"account already exists" banner wording.

CHANGELOG.md

@@ -10,7 +10,7 @@ and this project follows [Semantic Versioning](https://semver.org/).
 ### Fixed
 
 - Self-service kiosk: closed events no longer appear in the event picker — only open events are selectable.
-- Self-service kiosk: entering an email or phone number that matches an existing account now autofills the visitor's name, the other contact channel, and notification preference, instead of requiring staff to re-enter details already on file.
+- Self-service kiosk: added a "Look up existing account" search field (by email or phone, triggered only by Enter or the Search button — never as-you-type) that autofills a returning visitor's name, contact details, and notification preference from their exact matching account, instead of requiring staff to re-enter details already on file.
 
 ## [1.3.0] - 2026-07-27