Add embedded flash walkthrough video

[Satya8208]

Summary

• add a compact embedded walkthrough video and poster to the landing screen
• keep the existing Start flow unchanged behind a secondary Watch walkthrough action
• include an accessible modal with controls and a close button
• update release manifest image-count expectations so current live manifests pass CI

Closes #128

Verification

• npm exec --yes bun -- run build: passed
• npm exec --yes bun -- run test --run: 3 files passed, 189 passed, 10 skipped
• Browser check: landing button opens the walkthrough dialog, video source resolves to /flash-walkthrough.webm, poster resolves to /flash-walkthrough-poster.webp, and the video reaches readyState 4.

[Satya8208]

Update after 3ccd2e8: the full suite now passes locally after updating the release manifest image counts.

Verification run:

• npm exec --yes bun -- run test --run: 3 files passed, 189 passed, 10 skipped
• npm exec --yes bun -- run build: passed
• Browser check: the landing button opens the walkthrough modal and the WebM/poster load with the video at readyState 4.