fix: clear recipient check loading state on RPC timeout

[Bellamzy]

fix: clear recipient check loading state on RPC timeout

Problem

When the RPC provider timed out or hung during the on-chain recipient existence check, the
recipientStatus state was left at 'checking' indefinitely. This kept the submit button stuck
showing "Verifying recipient…" with no way for the user to proceed or recover — they had to
reload the page.

Two secondary issues existed alongside it:

• If the user changed the address while a check was already in-flight, the stale result could
  land after the new check started, incorrectly flipping the status.
• The component had no cleanup on unmount, so a pending check could call setRecipientStatus on
  an unmounted component.

Changes

lib/soroban.ts

• checkRecipientExists now accepts an optional { signal?, timeoutMs? } parameter.
• The underlying getAccount() call is wrapped in the existing withTimeout helper (default 10
  s), so a hung RPC provider always causes a rejection rather than an eternal pending promise.
• OperationAbortedError is re-thrown explicitly so callers can distinguish cancellation from a
  genuine network failure and skip stale state updates.

app/create/page.tsx

• The useEffect creates a new AbortController on each run.
• Cleanup aborts any in-flight check and immediately resets recipientStatus to 'idle', covering
  both address-change and component-unmount cases.
• A hard 10 s setTimeout inside the debounce callback aborts the controller if the RPC never
  responds, ensuring the spinner is always cleared regardless of network behaviour.
• State updates are guarded by controller.signal.aborted to prevent stale results from a
  superseded check from landing.

Testing

• Simulate a hung RPC (e.g. block the Soroban endpoint in DevTools → Network): status
  transitions to 'error' after 10 s and the button becomes usable again.
• Change the address rapidly while a check is in-flight: no stale 'valid'/'not-found' flash.
• Normal happy path (account exists / not found) unchanged.
  Bug: Token Selector hangs infinitely #123