fix: gate refund button on funder identity and add funderAddress field (#82)

[rafaio1]

Fixes #82.

• Add funderAddress field to Bounty type and map it through adaptBounty from the backend's RawBounty shape, making sponsor identity available to the frontend for authorization checks.
• Gate the "Refund sponsor" button rendering on user?.stellarAddress === bounty.funderAddress, so only the actual funder sees the action — preventing unrelated authenticated visitors from being shown a button they cannot legitimately use.
• Document that handleFund is intentionally unconditional (any sponsor can fund an open bounty per the crowdfunding model), distinguishing it from the restricted refund action via an explicit code comment.
• Backend authorization note: This fix addresses the frontend UX/trust gap regardless of backend behavior. If the backend already 403s non-funder refund attempts, this makes the UI consistent with that enforcement. If not, this prevents the frontend from ever sending an unauthorized request in the first place.
• TypeScript compiles cleanly with npx tsc --noEmit.

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