feat: booking confirm modal with cascading Hall → Trainer → Group, linear workflow

- Add confirmed_date, confirmed_group, confirmed_comment to group_bookings (migration #10)
- Linear booking flow: Новая → Связались → Подтвердить (modal) / Отказ
- Confirm modal with cascading selects: Hall → Trainer → Group → Date → Comment
- Groups merged by groupId — shows all days/times (e.g. "СР 20:00, ПТ 16:15")
- Auto-prefill hall/trainer/group from booking's groupInfo via fuzzy scoring
- Proper SHORT_DAYS constant for weekday abbreviations
- Filter chips with status counts, declined sorted to bottom
- "Вернуть" on confirmed/declined returns to "Связались" (not "Новая")

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/app/api/admin/group-bookings/route.ts

@@ -24,12 +24,12 @@ export async function PUT(request: NextRequest) {
       return NextResponse.json({ ok: true });
     }
     if (body.action === "set-status") {
-      const { id, status, confirmedDate } = body;
+      const { id, status, confirmation } = body;
       const valid: BookingStatus[] = ["new", "contacted", "confirmed", "declined"];
       if (!id || !valid.includes(status)) {
         return NextResponse.json({ error: "id and valid status are required" }, { status: 400 });
       }
-      setGroupBookingStatus(id, status, confirmedDate);
+      setGroupBookingStatus(id, status, confirmation);
       return NextResponse.json({ ok: true });
     }
     return NextResponse.json({ error: "Unknown action" }, { status: 400 });