fix(bounty): show working state and PR guidance to claimant on claimed bounty

[dezzy-music]

Fixes #699

Summary

When a bounty status was claimed, ClaimButton previously showed "This bounty has already been claimed" / "Claimed by: ..." indiscriminately to all visitors, including the claimant themselves. The contributor doing the work was presented with messaging designed for lost race participants, with no actionable guidance on linking their pull request.

Changes

• Compared bounty.claimedById with authenticated user.id (falling back to username comparison when claimedById is absent).
• Rendered a "You're working on this bounty" state for the claimant with explicit guidance to include Fixes #<issue-number> in their pull request description to link their work and receive their automated escrow release upon merge.
• Maintained the existing "This bounty has already been claimed" message for non-claimant visitors.
• Added test coverage in ClaimButton.test.tsx verifying claimant detection via claimedById or claimedBy and proper rendering of PR instructions.

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