fix: gate bounty claiming on linked payout wallet (#72)

[rafaio1]

Fixes #72.

• Add a hard-block in handleClaim that checks user.stellarAddress before allowing the claim API call to proceed. If no payout wallet is linked, the action is blocked client-side with a clear error message explaining why (automatic payout release has no manual recovery path) and the user is redirected to /connect to link one.
• This mirrors the existing if (!user) auth gate pattern already present in the same function, and parallels the withWallet guard used by handleFund — but correctly checks the durable backend-linked address (user.stellarAddress) rather than requiring a live Freighter session at claim time, since those are different requirements at different points in the flow.
• The blocked state uses the existing error display channel, which is visually distinct from the pending (in-flight) state per the component's current UX contract.
• Decision rationale (hard block vs. soft warning): Hard block chosen because the README documents fully-automatic payout release with no manual approval step, and BountyStatus has no state representing 'claimed but contributor has no payout wallet.' A soft warning would allow contributors to create an unrecoverable dead-end state where funds have nowhere to go after merge. Until a manual recovery flow or new status exists, preventing the action entirely is the safer default.
• Team-split audit: Out of scope for this fix, but noted for follow-up — if team splits allow naming contributors who haven't linked wallets, the same dead-end risk applies at payout time. No such check exists today.
• TypeScript compiles cleanly with npx tsc --noEmit.

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