`display` was initialised once and never updated, so the text box would
show a stale German date after the parent reset `value` (e.g. × reset
button or timeline drag). A guarded `$effect` re-derives `display` from
`value` whenever the two are out of sync while preserving mid-typing
partial dates (germanToIso returns '' for incomplete input, which matches
value='' during typing → no spurious re-derive).
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
