fix(PersonTypeahead): sync searchTerm when initialName prop changes

After a person swap the parent navigates to a new URL and the server
returns swapped names. The component's searchTerm was only set once from
initialName at mount time ($state(initialName) captures the initial value
only). Adding a reactive $effect ensures the displayed name updates
whenever initialName changes — fixing the swap button showing stale names.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/lib/components/PersonTypeahead.svelte

@@ -30,8 +30,16 @@ let {
 	onfocused
 }: Props = $props();
 
+// searchTerm must be both prop-derived AND locally writable (user typing), so $state +
+// $effect is the correct pattern here — writable $derived is read-only and won't work.
+// eslint-disable-next-line svelte/prefer-writable-derived
 let searchTerm = $state(initialName);
 
+// Sync display text when the selected person changes externally (e.g. swap, navigation).
+$effect(() => {
+	searchTerm = initialName;
+});
+
 $effect(() => {
 	const suggested = suggestedName;
 	if (suggested && !untrack(() => value)) {