- Add freeze/start/close action buttons on MilestoneDetailPage
- Freeze: visible in open status, calls POST .../actions/freeze
- Start: visible in freeze status, calls POST .../actions/start
- Close: visible in open/freeze/undergoing, with reason input + confirmation
- Display started_at in milestone meta when present
- Hide edit button and create-item buttons in terminal states
- Add CSS badge styles for freeze (purple), undergoing (amber), completed (green)
- All actions show loading state and error feedback
- types/index.ts: Task status 'progressing' → 'undergoing' + 'completed'; Milestone status updated to open/freeze/undergoing/completed/closed + started_at field
- MilestoneFormModal: dropdown options updated
- MilestoneDetailPage: isProgressing → isUndergoing, badge class simplified
- MilestonesPage: badge class simplified
- TaskDetailPage: status transition map updated for new state machine