Implement event completion workflow (Task 14)

- Add completeEvent endpoint to backend API
- Create EventCompletionForm component with litter quantity input and photo upload
- Update EventDetailModal to show completion form for organizers
- Add isOrganizer check in EventsPage
- Update event service to support completion
- Display completion data (litter collected, photos) for completed events

server/src/routes/eventRoutes.ts

@@ -16,5 +16,6 @@ router.delete('/:id', requireAuth, eventController.deleteEvent);
 router.post('/:id/register', requireAuth, eventController.registerForEvent);
 router.delete('/:id/register', requireAuth, eventController.unregisterFromEvent);
 router.get('/:id/registration-status', requireAuth, eventController.getRegistrationStatus);
+router.post('/:id/complete', requireAuth, eventController.completeEvent);
 
 export default router;