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/components/PitchDeck.tsx

@@ -251,6 +251,7 @@ export default function PitchDeck({ lang, onToggleLanguage }: PitchDeckProps) {
         onResume={presenter.resume}
         onStop={presenter.stop}
         onSkip={presenter.skipSlide}
+        onPrev={presenter.prevSlide}
         isSpeaking={presenter.isSpeaking}
         ttsAvailable={presenter.ttsAvailable}
         ttsEnabled={presenter.ttsEnabled}

pitch-deck/components/presenter/PresenterOverlay.tsx

@@ -1,7 +1,7 @@
 'use client'
 
 import { motion, AnimatePresence } from 'framer-motion'
-import { Play, Pause, Square, SkipForward, Volume2, VolumeX } from 'lucide-react'
+import { Play, Pause, Square, SkipForward, SkipBack, Volume2, VolumeX } from 'lucide-react'
 import { Language } from '@/lib/types'
 import { PresenterState } from '@/lib/presenter/types'
 import { SLIDE_ORDER } from '@/lib/hooks/useSlideNavigation'
@@ -18,6 +18,7 @@ interface PresenterOverlayProps {
   onResume: () => void
   onStop: () => void
   onSkip: () => void
+  onPrev?: () => void
   isSpeaking?: boolean
   ttsAvailable?: boolean
   ttsEnabled?: boolean
@@ -35,6 +36,7 @@ export default function PresenterOverlay({
   onResume,
   onStop,
   onSkip,
+  onPrev,
   isSpeaking,
   ttsAvailable,
   ttsEnabled = true,
@@ -108,6 +110,15 @@ export default function PresenterOverlay({
                       </button>
                     )}
 
+                    <button
+                      onClick={onPrev}
+                      className="w-7 h-7 rounded-full bg-white/10 flex items-center justify-center
+                        hover:bg-white/20 transition-colors"
+                      title={lang === 'de' ? 'Vorherige Folie' : 'Previous slide'}
+                    >
+                      <SkipBack className="w-3.5 h-3.5 text-white/60" />
+                    </button>
+
                     <button
                       onClick={onSkip}
                       className="w-7 h-7 rounded-full bg-white/10 flex items-center justify-center