Fix user dashboard: buried edit errors, closed events not hidden, closed/past registrations editable

Registration-edit errors were set into a page-level error state rendered
behind the edit modal overlay; they now render inside the modal. The
"Show past events" toggle only hid date-based past events, letting
cashup-closed events leak through by default; a shared isEventOver()
check now covers both, applied to registrations, tickets, and the
Edit button visibility (mirroring the backend's own edit block).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/src/controllers/registrationController.js

@@ -415,7 +415,7 @@ const getUserRegistrations = async (req, res) => {
     const whereClause = showPast ? { userId: req.user.id } : {
       userId: req.user.id,
       status: { not: 'cancelled' },
-      event: { endDate: { gte: now } }
+      event: { endDate: { gte: now }, cashupStatus: { not: 'closed' } }
     };
 
     const registrations = await prisma.registration.findMany({