fix: remove floating booking button overlapping mobile menu, center Open Day heading

- Remove floating "Записаться" button that covered hamburger menu on mobile scroll
- Booking still accessible via mobile menu dropdown and Hero CTA
- Center Open Day section heading to match all other sections

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

src/components/sections/OpenDay.tsx

@@ -49,7 +49,7 @@ export function OpenDay({ data }: OpenDayProps) {
     <section id="open-day" className="py-10 sm:py-14">
       <div className="mx-auto max-w-6xl px-4">
         <Reveal>
-          <SectionHeading>{event.title}</SectionHeading>
+          <SectionHeading centered>{event.title}</SectionHeading>
         </Reveal>
 
         <Reveal>