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/McRegistrationsTab.tsx

@@ -27,17 +27,17 @@ export function McRegistrationsTab({ filter }: { filter: BookingFilter }) {
       const dates: Record<string, string> = {};
       const mcItems = mcData.items || [];
       for (const mc of mcItems) {
-        const latestSlot = mc.slots?.reduce((latest, s) => s.date > latest ? s.date : latest, "");
-        if (latestSlot) dates[mc.title] = latestSlot;
+        const earliestSlot = mc.slots?.reduce((min, s) => s.date < min ? s.date : min, mc.slots[0]?.date ?? "");
+        if (earliestSlot) dates[mc.title] = earliestSlot;
       }
       const regTitles = new Set(regData.map((r) => r.masterClassTitle));
       for (const regTitle of regTitles) {
         if (dates[regTitle]) continue;
         for (const mc of mcItems) {
-          const latestSlot = mc.slots?.reduce((latest, s) => s.date > latest ? s.date : latest, "");
-          if (!latestSlot) continue;
+          const earliestSlot = mc.slots?.reduce((min, s) => s.date < min ? s.date : min, mc.slots[0]?.date ?? "");
+          if (!earliestSlot) continue;
           if (regTitle.toLowerCase().includes(mc.title.toLowerCase()) || mc.title.toLowerCase().includes(regTitle.toLowerCase())) {
-            dates[regTitle] = latestSlot;
+            dates[regTitle] = earliestSlot;
             break;
           }
         }