fix(buchung): Plus-Button mobil unten mittig

Der schwebende Plus-Button sass rechts unten und verdeckte den
Kopier-Knopf der Buchungskarten. Jetzt mittig, dazu Abstand am
Seitenende, damit die letzte Karte ueber den Button scrollen kann.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/components/booking/Booking.tsx

@@ -140,7 +140,7 @@ export const Booking: React.FC = () => {
   );
 
   return (
-    <div className="mx-auto max-w-[1500px]">
+    <div className={`mx-auto max-w-[1500px] ${isMobile && !readOnly ? 'pb-16' : ''}`}>
       {/* Werkzeugleiste */}
       <div className="mb-4 flex flex-wrap items-center gap-2 sm:gap-3">
         <div className="flex items-center rounded-xl border border-ink-200 bg-surface shadow-card">
@@ -276,7 +276,7 @@ export const Booking: React.FC = () => {
         <button
           onClick={() => openNew()}
           aria-label="Zeit buchen"
-          className="animate-glow fixed bottom-24 right-5 z-40 flex h-16 w-16 items-center justify-center rounded-full bg-brand-500 text-white shadow-lift transition active:scale-95"
+          className="animate-glow fixed bottom-24 left-1/2 z-40 flex -translate-x-1/2 h-16 w-16 items-center justify-center rounded-full bg-brand-500 text-white shadow-lift transition active:scale-95"
         >
           <Icon name="plus" className="h-8 w-8" strokeWidth={2.6} />
         </button>