Add optimistic bounty status updates

[pntech20]

Summary

• add a Zustand-backed bounty store with 3-second mock Stellar confirmations and a 20% failure rollback path
• add pending and optimistic status fields to bounty data so cards and detail views update immediately
• wire the marketplace, my-bounties, and detail pages to the store actions for accepting and submitting bounty work

Checks

• npx tsc --noEmit -p frontend/tsconfig.json
• npm run lint --prefix frontend
• npm run build --prefix frontend
• git diff --check

Closes #212