test: verify recipient check debounce and abort behavior

[ZacLou]

Summary

Addresses #466

Context

The issue asks to confirm that checkRecipientExists still fires once per 600ms of idle (not on every keystroke) and that the in-flight check is aborted when the address changes.

After reviewing the code, the debounce is correctly implemented:

• useDebounce(recipient, 600) debounces the recipient value
• The effect depends on debouncedRecipient, not recipient
• An AbortController is created per effect run with cleanup that aborts on address change
• The isMounted flag prevents stale state updates

Tests Added

3 new test cases in app/create/__tests__/page.test.tsx:

1. Does not call checkRecipientExists immediately on keystroke — verifies the mock is not called until the 600ms debounce fires
2. Fires only one check after rapid keystrokes — types 5 keystrokes within the debounce window, verifies only one check fires
3. Aborts the in-flight check when the address changes mid-debounce — types first address, waits for debounce, changes address, verifies only the new address triggers a check

No production code changes

The implementation is correct — these tests provide regression coverage to prevent future debounce breakage.