style: flat docs borders, framed home hero, and button-group edge fixes

[yyyyaaa]

Summary

Visual fine-tuning on top of the redesign.

• Home hero — now a square, flat panel inset by the page gutter, so its edges line up exactly with the showcase grid below (previously its overlays filled the wider content column and stopped dead at the edge). 1px --border, no shadow, overlays clipped to the frame.
• Docs chrome — every surface that relied on the faint shadow hairline uses an explicit --border: example frames and their bars, catalog and page tiles (AI / billing / features / command palette), code blocks, API table, install toggle, preset tiles, preview canvas. Tile hovers tint instead of lifting.
• ButtonGroup — the last child lost its outer corners because rounded-r-none applied to every child; selectors now square only the shared edges (:not(:first-child) / :not(:last-child), Button before: overlay included), which holds for any child count. ButtonGroupSeparator had its width zeroed by an override on the wrong axis and sat in a slot that showed the page through; it is now a 1px translucent ink line drawn over the preceding edge, so it tints whatever fill it divides. Unit test pins the selectors.

Visual

Surface	Before	After
Home hero	overlays spanning the full content column, rounded, shadowed	square flat panel on the grid's edges, 1px border
Docs example frame	shadow hairline (near-invisible in dark), 60% bar divider	1px border on all sides, full-strength bar divider
Split button	last button square on the outside; 3px page-coloured gap at the divider	rounded outer corners; 1px tinted divider, no gap

Generated with Devin

[tenki-reviewer]

Review complete. No issues found — approved ✅.

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The change spans the docs app pages and shared docs components, plus a new UI component. It tightens borders and gutters across the registry chrome, reshapes page metadata (openGraph/OG_IMAGE), and refactors CodeBlock, api-table, catalog-card, install-toggle, primitive-preview, and theme-preset-grid. A new ButtonGroup component with corner-squaring edge selectors and separator ink lines was added along with a regression test.

Files	Change
apps/blocks/src/app/globals.css	Swaps hero/block box-shadow for borders and adjusts gutter margins across breakpoints.
apps/blocks/src/app/blocks/{ai,billing,command-palette,features}/page.tsx	Updates page metadata (openGraph/OG_IMAGE) and trailing-comma formatting.
apps/blocks/src/components/docs/{code-block,api-table,catalog-card,install-toggle,primitive-preview,theme-preset-grid}.tsx	Refactors shared docs components (highlight rendering, table/API props, install toggle state, preset grid).
packages/ui/src/components/button-group.tsx	Adds ButtonGroup with shared-edge corner squaring, overlay highlight, and separator ink lines.
packages/ui/test/button-group.test.ts	Adds behavior tests asserting squared corners and border selectors.

All candidate findings fell below the confidence threshold; only low-confidence, unverified reachability notes were surfaced and none met the bar for an inline comment.

_{Reviewed commit: bf376fc}