feat(website): recorded demo videos for the homepage showcase (#633)

LangGraph (demo.threadplane.ai) and AG-UI (ag-ui.threadplane.ai) each captured
running a real a2ui feedback-form render via Playwright, encoded small
(mp4 + webm + first-frame webp poster, ~460KB total). Fills the DemoShowcase
video-first tabs added in #632.

Also drop the stale apps/website/public/demo/ ignore (added alongside
test-results/cache dirs in 1479a03) so these intentional public assets are
tracked and deployed.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: blove

.gitignore

@@ -14,7 +14,6 @@ deploy/
 .nx/workspace-data
 tmp/
 apps/website/test-results/
-apps/website/public/demo/
 cockpit/**/angular/test-results/
 /test-results/