fix(date-input): re-derive display when value prop changes externally

`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>

frontend/src/lib/shared/primitives/DateInput.svelte

@@ -1,4 +1,5 @@
 <script lang="ts">
+import { untrack } from 'svelte';
 import { isoToGerman, handleGermanDateInput, germanToIso } from '$lib/shared/utils/date';
 import { m } from '$lib/paraglide/messages.js';
 
@@ -24,6 +25,16 @@ let {
 
 let display = $state(isoToGerman(value ?? ''));
 
+// Re-derive display when value changes externally (e.g. timeline drag, reset nav).
+// Guard prevents overwriting while the user is mid-typing a partial date:
+// germanToIso returns '' for partial input, matching value '' → no re-derive.
+$effect(() => {
+	const externalIso = value ?? '';
+	if (germanToIso(untrack(() => display)) !== externalIso) {
+		display = isoToGerman(externalIso);
+	}
+});
+
 // ─── Validation helper ────────────────────────────────────────────────────
 function isCalendarValid(iso: string): boolean {
 	if (!iso) return false;