feat: Presenter Vor/Zurück-Spulen mit Folien-Sync

- prevSlide() in usePresenterMode: springt zur vorherigen Folie,
  stoppt aktuelle Audio, startet Präsentation der vorherigen Folie
- SkipBack Button in PresenterOverlay neben SkipForward
- Beide Buttons springen zur korrekten Folie UND starten die Audio

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

pitch-deck/lib/hooks/usePresenterMode.ts

@@ -22,6 +22,7 @@ interface UsePresenterModeReturn {
   pause: () => void
   resume: () => void
   skipSlide: () => void
+  prevSlide: () => void
   toggle: () => void
   displayText: string
   progress: number
@@ -386,6 +387,20 @@ export function usePresenterMode({
     }
   }, [cancelAll, totalSlides, goToSlide])
 
+  const prevSlide = useCallback(() => {
+    cancelAll()
+    const prevIdx = slideIndexRef.current - 1
+    if (prevIdx >= 0) {
+      goToSlide(prevIdx)
+      slideIndexRef.current = prevIdx
+      if (stateRef.current === 'presenting' || stateRef.current === 'paused') {
+        setState('presenting')
+        stateRef.current = 'presenting'
+        playSlideRef.current(prevIdx)
+      }
+    }
+  }, [cancelAll, goToSlide])
+
   const toggle = useCallback(() => {
     unlockAudio()
     if (stateRef.current === 'idle') {
@@ -422,6 +437,7 @@ export function usePresenterMode({
     pause,
     resume,
     skipSlide,
+    prevSlide,
     toggle,
     displayText,
     progress,