fix: MC series uses earliest slot date for registration cutoff

Multi-session master classes are a series — once the first session
passes, the group has started and registration closes. Changed all
MC date logic from "latest slot" / "any future slot" to "earliest slot":

- DashboardSummary: upcoming = earliest slot >= today
- McRegistrationsTab: archive = earliest slot < today
- AddBookingModal: only show MCs where earliest slot >= today
- Public MasterClasses: isUpcoming checks earliest slot

src/app/admin/bookings/page.tsx

@@ -582,7 +582,8 @@ function DashboardSummary({ onNavigate }: { onNavigate: (tab: Tab) => void }) {
         // Build set of upcoming MC titles
         const upcomingTitles = new Set<string>();
         for (const mc of mcData.items || []) {
-          if (mc.slots?.some((s) => s.date >= today)) upcomingTitles.add(mc.title);
+          const earliest = mc.slots?.reduce((min, s) => s.date < min ? s.date : min, mc.slots[0]?.date ?? "");
+          if (earliest && earliest >= today) upcomingTitles.add(mc.title);
         }
         return regs.filter((r) => upcomingTitles.has(r.masterClassTitle));
       }),