chore(marketing): add final Product Hunt launch assets

[wizkidword]

What changed

• Added the final public guest-flow Product Hunt gallery, thumbnail, and social image.
• Added a gated 1920x1080 captioned H.264 demo video plus YouTube thumbnail.
• Updated capture tooling so the reader gallery slide captures a real article in Card layout.
• Updated launch status and manifest evidence to reflect the final validated package.

Validation

• npm run ph:assets
• npm run ph:assets:validate
• approved-capture npm run ph:package
• npm test (817 passed)
• npm run typecheck
• npm run lint (two pre-existing warnings; no errors)
• confirmed the demo is 1920x1080 H.264, 52 seconds, with fast-start MP4 metadata.

No production deployment is required for this asset-only change.