Add right-click context menu for contracts on contracts and board pages (#11)

Implement a reusable ContractContextMenu component that provides:
- Mark as Complete/Active/Archive status actions (conditionally shown)
- Go to Supervisor Task link (when supervisor exists)
- Delete action with confirmation

Integrate context menu into:
- ContractList.tsx on the contracts page
- WorkflowBoard on the workflow/board page via PhaseColumn and WorkflowContractCard

Features match ElementContextMenu patterns:
- Fixed positioning with z-50
- Click outside and Escape key close handlers
- Viewport overflow prevention
- Dark theme colors (#0a1628, #0d1b2d, #75aafc, #9bc3ff)

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

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