Add split payout support for multiple PR contributors

[mateoalifraco]

Summary

• Adds an opt-in bountic-split block in PR bodies so maintainers can distribute a locked bounty across multiple contributors by percentage or fixed USDC amounts.
• Pays each split recipient through the existing wallet/email/unclaimed payout resolution path and records separate payout/activity events.
• Updates the approval UI/API response shape so maintainers see when a split payout was sent.

Example PR body syntax:

<!-- bountic-split:
@alice 60% 0x1111111111111111111111111111111111111111
@bob 40%
-->

Fixed amounts are also supported as long as they total the bounty amount:

<!-- bountic-split:
@alice 6.00
@bob 4.00
-->

Verification

• tsc --noEmit
• eslint over the modified files:
  • lib/bounty/services/payout.ts
  • lib/bounty/services/approve-payout.ts
  • lib/api/client.ts
  • app/b/[owner]/[repo]/issues/[issueNumber]/approve-button.tsx

Related to #12.

[vercel]

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