Restrict event completion to after event date

- Only allow organizers to complete events after the scheduled date has passed
- Show informative message to organizers before event date
- Prevents premature event completion

.kiro/specs/beach-litter-mapping/tasks.md

@@ -108,7 +108,7 @@
   - Create event detail page with participant count
   - _Requirements: 4.1, 4.2, 4.3, 4.4_
 
-- [-] 14. Implement event completion workflow
+- [x] 14. Implement event completion workflow
   - Create event completion form for organizers
   - Add litter collected quantity input
   - Implement photo upload for cleanup results