feat(frontend): Add UI for phase transition confirmation requests

When phase_guard is enabled and a supervisor tries to advance the contract
phase, users now see a confirmation modal with:
- Current and proposed next phase visualization
- Phase deliverables checklist (if available)
- Summary of the phase work
- Options to "Approve & Advance" or "Request Changes" with feedback

Components added:
- PhaseConfirmationModal: Full modal dialog for phase confirmations
- PhaseConfirmationInline: Inline variant for task output view
- PhaseConfirmationNotification: Global notification wrapper
- PhaseConfirmationToast: Alternative toast-style notification

Integration:
- Added phase_confirmation message type to TaskOutput renderer
- Extended PendingQuestion API type with phase confirmation data
- Integrated notification into main app layout

The UI uses the existing supervisor question infrastructure (polling via
/api/v1/mesh/questions) and responds with APPROVE or CHANGES_REQUESTED
prefixed feedback.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

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