fix(transcription): cancel pending @mention debounce in onExit

Without this, a closed dropdown's trailing runSearch could fire against
the next dropdown's state and silently overwrite its items before its
own fetch resolved. Felix #1 on PR #629.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/src/lib/shared/discussion/PersonMentionEditor.svelte

@@ -268,6 +268,13 @@ onMount(() => {
 								return exports?.onKeyDown(event) ?? false;
 							},
 							onExit() {
+								// Cancel any pending debounce so a closed dropdown's trailing
+								// runSearch cannot fire against the *next* dropdown's state.
+								// The hoisted `cancelPendingSearch` would be overwritten by
+								// the next render()'s onStart before the trailing call fires,
+								// so we cancel locally via the closure-scoped debouncedSearch.
+								// Felix #1 on PR #629.
+								debouncedSearch.cancel();
 								if (mountedDropdown) {
 									unmount(mountedDropdown);
 									mountedDropdown = null;