feat(docs-site): lead the homepage with the 90-second overview, link the YouTube channel

[os-zhuang]

What

The README has carried the overview video since #11941 — a cover linking to https://youtu.be/CX_FlOoOtr0. The site homepage had nothing. It now plays there, in the position a product video is normally given: directly under the headline, before anything else.

The YouTube channel (https://www.youtube.com/@objectstack) is linked twice on the homepage — a YouTube button in the hero CTA row next to GitHub, and "More videos on YouTube" under the video.

How

• apps/docs/components/youtube-embed.tsx — a click-to-play facade, not a bare <iframe>. Nothing is requested from YouTube until the viewer clicks, so the homepage costs no third-party frame, script or cookie on load; the click swaps in a youtube-nocookie.com iframe with autoplay=1.
• The cover is the README's own hero-cover-dark.png, copied into apps/docs/public/. It is 2:1 while the video is 16:9, so the frame is a fixed 2:1 with the player centred inside it at its own aspect ratio. Cropping the cover to 16:9 cuts through the logo on its left edge, and re-shaping the frame on click would shove the page down mid-interaction.
• Hero restructured to a single centred column (eyebrow, h1, lead, CTAs, chips) with the video full-width beneath it. Beside a three-paragraph text column the video was small and floated in the middle of the row. The two paragraphs it displaced now sit under the video as a two-column band, and the artifact card moved down to "01 · The loop" — which is what it depicts. No copy was deleted.

Verification

Browser-verified against the running dev server at 1440 / 1280 / 500px, light and dark:

• poster renders, no horizontal overflow, no layout shift on play;
• click loads https://www.youtube-nocookie.com/embed/CX_FlOoOtr0?autoplay=1&rel=0 at 864×486 (16:9) and plays;
• both channel links resolve to https://www.youtube.com/@objectstack;
• pnpm --filter @objectstack/docs typecheck and eslint on the changed files are clean.

Docs-site only, no published package — no changeset, hence skip-changeset.

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