FE: Cancel / refund flow #634

Context

Part of the Bounty Operate to Payout milestone (v1 boundless). If a bounty gets no acceptable submissions (or the organizer aborts), they cancel it and the escrow refunds (contributors first, then owner residual).

Tasks

• Cancel action (in management settings/overflow): confirmation explaining the refund (contributor-first, then owner), require explicit confirm.
• Build the cancel escrow op, drive useEscrowOpRunner -> progress -> CANCELLED.
• Reflect CANCELLED state + refund tx.

Blueprint to mirror

• Hackathon cancel/refund escrow flow; the existing bounty POST .../escrow/cancel endpoint.

Acceptance criteria

• Organizer can cancel a published bounty; refund settles; bounty shows CANCELLED.

Depends on

• Bounty data layer (organizer escrow client cancel already exists).