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/AddBookingModal.tsx

@@ -42,10 +42,13 @@ export function AddBookingModal({
     adminFetch("/api/admin/sections/masterClasses").then((r) => r.json()).then((data: { items?: { title: string; slots: { date: string }[] }[] }) => {
       const today = new Date().toISOString().split("T")[0];
       const upcoming = (data.items || [])
-        .filter((mc) => mc.slots?.some((s) => s.date >= today))
+        .filter((mc) => {
+          const earliest = mc.slots?.reduce((min, s) => s.date < min ? s.date : min, mc.slots[0]?.date ?? "");
+          return earliest && earliest >= today;
+        })
         .map((mc) => ({
           title: mc.title,
-          date: mc.slots.reduce((latest, s) => s.date > latest ? s.date : latest, ""),
+          date: mc.slots.reduce((min, s) => s.date < min ? s.date : min, mc.slots[0]?.date ?? ""),
         }));
       setMcOptions(upcoming);
       if (upcoming.length === 0 && tab === "events") setEventType("open-day");