Expose TTL keeper actions in the frontend (renew-lease button + keeper status)

[gbengaeben]

Category: Frontend UX | Priority: Medium | Dependencies: none | Out of scope: moving the keeper into the frontend (stays backend); notifications (own issue)

Description & Context: SDK helpers bumpStreamTtl/bumpContractTtl exist but nothing calls them from the UI. Judges can't see the TTL story, and users have no fallback if the backend keeper is down.

Acceptance Criteria / DoD:

• New getRemainingTtl(streamId) in frontend/src/lib/contracts/stream-core.ts (reads the stream entry's liveUntilLedgerSeq via RPC getLedgerEntries, converts to an estimated days-remaining)
• Stream card shows a "Renew lease" button, disabled unless remaining TTL < 30 days, calling bumpStreamTtl; result surfaced via decodeSorobanError
• Dashboard footer shows last keeper pass from GET {BACKEND_URL}/status: ok/degraded + lastPass.ts, refreshed on an interval; degraded renders a warning banner
• tsc -b clean; reuses the error-decoder unit tests from the frontend-testing issue

How to verify: Against Testnet: create a stream, lower the TTL threshold via env, confirm the button appears and a bump tx lands; stop the keeper and confirm the degraded banner within one refresh interval.