fix(page-practice): debounce blur to prevent ad refresh resetting lesson

[faneb]

Summary

Fixes #168 — third-party ad scripts briefly steal and return focus, which triggers handleBlur → onResetLesson(), resetting the lesson mid-session.

• Debounce blur handling: handleBlur now delays the lesson reset by 300 ms. If focus returns within that window (as it does during an ad refresh), handleFocus cancels the pending timer and the lesson continues uninterrupted.
• Clear orphaned timers: handleBlur clears any existing timer before starting a new one, preventing multiple queued resets if blur fires rapidly. Timer is also cleaned up in componentWillUnmount.
• Add regression test: Renders the full PracticeScreen, simulates a transient blur/re-focus cycle, and asserts the lesson text is preserved.

[semanticdiff-com]

Review changes with  

Changed Files

File	Status
packages/page-practice/lib/practice/Presenter.tsx	21% smaller
packages/page-practice/lib/practice/Presenter.test.tsx	0% smaller