feat: Tag/Nacht-Modus fuer gesamtes Pitch Deck

- CSS-Variablen-basiertes Theming (globals.css)
- .theme-light Klasse auf html-Element schaltet alles um
- Toggle-Button oben rechts (Sonne/Mond Icon)
- Light Mode: helle Hintergruende, dunkle Texte, gedaempfte Glass-Effekte
- Alle text-white/* Klassen werden per CSS Override umgemapped
- Partikel-Background auf 8% Opacity im Light Mode
- Kein text-shadow-glow im Light Mode

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

pitch-deck/components/PitchDeck.tsx

@@ -52,6 +52,19 @@ export default function PitchDeck({ lang, onToggleLanguage }: PitchDeckProps) {
   const { data, loading, error } = usePitchData()
   const nav = useSlideNavigation()
   const [fabOpen, setFabOpen] = useState(false)
+  const [theme, setTheme] = useState<'dark' | 'light'>('dark')
+
+  const toggleTheme = useCallback(() => {
+    setTheme(prev => {
+      const next = prev === 'dark' ? 'light' : 'dark'
+      if (next === 'light') {
+        document.documentElement.classList.add('theme-light')
+      } else {
+        document.documentElement.classList.remove('theme-light')
+      }
+      return next
+    })
+  }, [])
 
   const presenter = usePresenterMode({
     goToSlide: nav.goToSlide,
@@ -172,10 +185,27 @@ export default function PitchDeck({ lang, onToggleLanguage }: PitchDeckProps) {
   }
 
   return (
-    <div className="h-screen relative overflow-hidden bg-gradient-to-br from-slate-950 via-[#0a0a1a] to-slate-950">
+    <div className={`h-screen relative overflow-hidden bg-gradient-to-br ${theme === 'light' ? 'from-[#eef0f5] via-[#f8f9fc] to-[#eef0f5]' : 'from-slate-950 via-[#0a0a1a] to-slate-950'}`}>
       <ParticleBackground />
       <ProgressBar current={nav.currentIndex} total={nav.totalSlides} />
 
+      {/* Theme Toggle */}
+      <button
+        onClick={toggleTheme}
+        className="fixed top-4 right-4 z-50 p-2 rounded-full bg-white/[0.08] border border-white/10 hover:bg-white/[0.15] transition-colors backdrop-blur-xl"
+        title={theme === 'dark' ? 'Tag-Modus' : 'Nacht-Modus'}
+      >
+        {theme === 'dark' ? (
+          <svg className="w-4 h-4 text-amber-300" fill="none" viewBox="0 0 24 24" stroke="currentColor" strokeWidth={2}>
+            <circle cx="12" cy="12" r="5" /><path d="M12 1v2m0 18v2M4.22 4.22l1.42 1.42m12.72 12.72l1.42 1.42M1 12h2m18 0h2M4.22 19.78l1.42-1.42M18.36 5.64l1.42-1.42" />
+          </svg>
+        ) : (
+          <svg className="w-4 h-4 text-indigo-400" fill="none" viewBox="0 0 24 24" stroke="currentColor" strokeWidth={2}>
+            <path d="M21 12.79A9 9 0 1111.21 3 7 7 0 0021 12.79z" />
+          </svg>
+        )}
+      </button>
+
       <SlideContainer slideKey={nav.currentSlide} direction={nav.direction}>
         {renderSlide()}
       </SlideContainer>