feat: add X Ads download conversion tracker to DownloadAdDialog

[nahcnuj]

��## Summary
Add X (Twitter) Ads conversion tracking specifically for downloads.

• Fires the provided X conversion event on download button clicks inside DownloadAdDialog.
• The base pixel script is only included on pages that contain download content (downloadAdPopup: true via the download link plugin).
• Non-download pages (index, regular essays, diary posts, etc.) receive no X tracker code.

Changes

• app/components/DownloadAdDialog.tsx: Extended setupDownloadAdPopup with optional xPixelFn.
• app/client.ts: Added xPixelFn (prod: calls twq, dev: logs; includes safe queuing stub).
• app/components/RootLayout.tsx: Added conditional X base code snippet (only PROD && downloadAdPopup).
• app/components/DownloadAdDialog.test.ts: Added tests for the new X pixel firing path (both alone and together with GA).

Verification

• All unit tests pass (144/144).
• Full production build succeeds.
• Confirmed via built HTML:
  • Download pages (/essays/download-link*, /works/download-link-test) contain the X base code.
  • All other pages have zero occurrences of twq, ov0j6, or "X conversion tracking".

This is a non-visual / analytics-only change (no screenshots needed per AGENTS.md).

Code installed

<!-- X conversion tracking event code -->
<script type="text/javascript">
  // Insert X Event ID
  twq('event', 'tw-ov0j6-ov0j9', {});
</script>
<!-- End X conversion tracking event code -->

(The base code is loaded once per qualifying page; the event is fired from the JS download handler.)