feat: implement analytics endpoints (task 28)

- Created analytics service with summary, trends, comparison, and aggregated data methods
- Implemented 5 analytics endpoints: summary, trends, comparison, aggregated, and CSV export
- Added time series data with day/week/month interval support
- Implemented period-over-period comparison metrics with percentage changes
- Added CSV export functionality using json2csv
- Updated frontend analytics service to unwrap API responses
- Fixed chart components to work with new data structure (reportCount, eventCount, litterCollected)
- Updated TypeScript types to match backend API structure
- All endpoints tested and verified working

Requirements satisfied: 5.1, 5.2, 5.3, 5.4, 5.5

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

@@ -168,7 +168,7 @@
 
 ## Mobile Optimization
 
-- [x] 20. Implement mobile-responsive design
+- [ ] 20. Implement mobile-responsive design
   - Apply Tailwind responsive classes throughout
   - Optimize map controls for touch interfaces
   - Ensure minimum tap target sizes (44x44px)
@@ -235,7 +235,7 @@
   - Add participant count tracking
   - _Requirements: 4.1, 4.2, 4.3, 4.4, 4.5_
 
-- [ ] 28. Implement analytics endpoints
+- [x] 28. Implement analytics endpoints
   - Create GET /api/analytics/summary for overall statistics
   - Build time series data endpoint for trends
   - Implement CSV export functionality