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/components/player/AudioPlayer.tsx

@@ -115,7 +115,7 @@ export default function AudioPlayer() {
       : `${m}:${sec.toString().padStart(2, '0')}`
   }
 
-  const currentChapter = chapters.findLast?.((c: any) => currentTime >= c.start) || chapters[0]
+  const currentChapter = [...chapters].reverse().find((c: any) => currentTime >= c.start) || chapters[0]
 
   const addBookmark = async () => {
     if (!item) return