fix: TypeScript-Fehler im Frontend beheben

- AudioPlayer: findLast → reverse().find() (ES2022 Kompatibilität)
- ChapterList: findLastIndex → manuelles for-loop + implizit any behoben
- Library: searchDebounce Variable 't' undefined → korrekte Initialisierung

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

frontend/src/pages/Library.tsx

@@ -50,8 +50,8 @@ export default function Library() {
 
   const searchDebounce = useCallback(
     (() => {
-      let t: ReturnType<typeof setTimeout>
-      return (v: string) => { clearTimeout(t); setSearch(v); setPage(0) }
+      let t: ReturnType<typeof setTimeout> | undefined
+      return (v: string) => { clearTimeout(t); t = setTimeout(() => { setSearch(v); setPage(0) }, 300) }
     })(),
     []
   )