feat(transcription): drive @mention fetch through the dropdown search input

For issue #380 (AC-2, AC-3, AC-4 + NFR debounce).

The search input is now the single fetch trigger. The dropdown's
searchQuery reactivity calls onSearch on every change — whether sourced
from the editor mirror or the user's own input. PersonMentionEditor
debounces these calls at 150 ms, short-circuits on empty queries (no
fetch, items cleared), and tears down pending timers on destroy.

The Tiptap suggestion plugin's items() now returns [] — per-keystroke
fetches in the editor are gone. The same /api/persons?q= endpoint is
used; the difference is in when and how often the request fires.

Adds a cancel() method to the debounce utility so destroyed editors
don't leave trailing fetches alive (which previously polluted the test
ledger and would have wasted bandwidth in production tab-close races).

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

frontend/src/lib/shared/utils/debounce.ts

@@ -1,12 +1,25 @@
 /**
  * Returns a debounced version of fn that delays invocation until after
- * `delay` ms have elapsed since the last call.
+ * `delay` ms have elapsed since the last call. The returned function
+ * exposes a `cancel()` method that clears any pending invocation —
+ * essential when the host context (a destroyed component, an unmounted
+ * editor) shouldn't fire the trailing call.
  */
 // eslint-disable-next-line @typescript-eslint/no-explicit-any
-export function debounce<T extends (...args: any[]) => void>(fn: T, delay: number): T {
-	let timer: ReturnType<typeof setTimeout>;
-	return ((...args: Parameters<T>) => {
-		clearTimeout(timer);
+export function debounce<T extends (...args: any[]) => void>(
+	fn: T,
+	delay: number
+): T & { cancel: () => void } {
+	let timer: ReturnType<typeof setTimeout> | undefined;
+	const wrapped = ((...args: Parameters<T>) => {
+		if (timer !== undefined) clearTimeout(timer);
 		timer = setTimeout(() => fn(...args), delay);
-	}) as T;
+	}) as T & { cancel: () => void };
+	wrapped.cancel = () => {
+		if (timer !== undefined) {
+			clearTimeout(timer);
+			timer = undefined;
+		}
+	};
+	return wrapped;
 }