feat: centralize popup texts in new admin tab /admin/popups

- New admin page for shared popup texts (success, waiting list, error, Instagram hint)
- Removed popup fields from MC and Open Day admin editors
- All SignupModals now read from centralized popups config
- Stored as "popups" section in DB with fallback defaults

src/app/page.tsx

@@ -30,7 +30,7 @@ export default function HomePage() {
       <Header />
       <main>
         <Hero data={content.hero} />
-        {openDayData && <OpenDay data={openDayData} />}
+        {openDayData && <OpenDay data={openDayData} popups={content.popups} />}
         <About
           data={content.about}
           stats={{
@@ -41,7 +41,7 @@ export default function HomePage() {
         />
         <Team data={content.team} schedule={content.schedule.locations} />
         <Classes data={content.classes} />
-        <MasterClasses data={content.masterClasses} regCounts={mcRegCounts} />
+        <MasterClasses data={content.masterClasses} regCounts={mcRegCounts} popups={content.popups} />
         <Schedule data={content.schedule} classItems={content.classes.items} />
         <Pricing data={content.pricing} />
         <News data={content.news} />