Support multi-contributor bounty payouts

[Friendiks-codex]

Summary

• Adds optional multi-contributor payout distribution via a hidden bountic-split block in the merged PR body.
• Supports either percentage splits that total 100% or fixed USDC splits that total the bounty amount.
• Persists one payout event and one activity event per recipient while preserving the existing single-recipient payout path.
• Updates the approval UI to show a split payout success message when multiple recipients are paid.

Split format

<!-- bountic-split:
@alice 60% 0x1111111111111111111111111111111111111111
@bob 40% 0x2222222222222222222222222222222222222222
-->

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

Test plan

• npx pnpm run typecheck
• npx eslint 'app/b/[owner]/[repo]/issues/[issueNumber]/approve-button.tsx' lib/api/client.ts lib/bounty/services/approve-payout.ts lib/bounty/services/payout.ts

Note: repo-wide npx pnpm run lint still reports pre-existing unrelated lint errors in app/mock-checkout/[checkoutId]/page.tsx and app/page.tsx.

Closes #12.

[bountic-bot]

⚡️ Bounty Competition Started

@Friendiks-codex has submitted a PR that references an issue with a bounty. Learn more about this bounty: https://bountic.vercel.app/b/skndash96/bountic/issues/12

When this PR is merged, the bounty will be locked and ready for payout approval.

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Bountic: Autonomous USDC bounties for open source

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