[WIP] choseisan-ical の実装タスク

[Copilot]

• Explore existing codebase structure and understand current implementation
• Verify build process works correctly
• Understand CSV parsing and validation logic already implemented
• Implement iCal generation utility function
• Add backend API endpoint for iCal generation
• Update frontend to submit form data and handle iCal response
• Add proper Japanese date/time parsing for iCal format
• Test complete workflow with sample data
• Deploy and verify webcal functionality

Current state: The frontend form and CSV parsing logic are already implemented. Need to add iCal generation backend functionality and integrate with frontend.

Fixes #45.

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