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/components/sections/MasterClasses.tsx

@@ -72,16 +72,18 @@ function calcDuration(slot: MasterClassSlot): string {
 
 function isUpcoming(item: MasterClassItem): boolean {
   const now = new Date();
-  return (item.slots ?? []).some((s) => {
-    const slotDate = parseDate(s.date);
-    if (s.startTime) {
-      const [h, m] = s.startTime.split(":").map(Number);
-      slotDate.setHours(h, m, 0, 0);
-    } else {
-      slotDate.setHours(23, 59, 59, 999);
-    }
-    return slotDate > now;
-  });
+  const slots = item.slots ?? [];
+  if (slots.length === 0) return false;
+  // Series MC: check earliest slot — if first session passed, group already started
+  const earliestSlot = slots.reduce((min, s) => s.date < min.date ? s : min, slots[0]);
+  const d = parseDate(earliestSlot.date);
+  if (earliestSlot.startTime) {
+    const [h, m] = earliestSlot.startTime.split(":").map(Number);
+    d.setHours(h, m, 0, 0);
+  } else {
+    d.setHours(23, 59, 59, 999);
+  }
+  return d > now;
 }
 
 function MasterClassCard({