feat: polish Blocks UI and motion

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Summary

• polish the full Blocks documentation shell, landing showcase, auth/org/shell/chat blocks, shared UI primitives, and schema builder surfaces
• standardize 40px desktop and 44px touch targets, .96 press feedback, explicit transitions, contextual icon swaps, and reduced-motion behavior
• improve mobile safe-area layout, overflow containment, typography wrapping, concentric radii, layered surfaces, and media separation
• pause expensive visual effects offscreen or when motion is reduced, guard WebGL capability, and remove hydration-prone initial presence animations
• regenerate the canonical Blocks manifest and refresh all 15 reviewed visual baselines without changing the 157-item public registry contract

No automated npm publishing is added or changed.

Verification

• pnpm check
• pnpm build:pages — 188 static pages generated
• pnpm pack:local
• pnpm --filter blocks test:visual — 16/16 passed
• clean-server browser matrix across landing, catalog, sign-in, chat, and schema builder in desktop/mobile, light/dark, and reduced motion
• residual scans for transition-all, layout-property transitions, unsafe initial presence animation, stale h-screen, arbitrary numeric z-indexes, and undersized press scales