feat(website): dark mode, header redesign, spec-sheet metrics, content refresh - #17

What

Full landing-page refresh, iterated live with the owner on the dev server:
site-wide light/dark mode with a shared toggle, header redesign (no pill morph; brand left, nav+actions right), capabilities reworked into a spec-sheet layout, a new "Beyond a single diff" section, and content updates reflecting the current product (RSR, native LangGraph/CrewAI ingestion, suites, A/B mode, adapter registry).

Why

Owner-directed website pass before the 0.3.0 release: the page predated RSR, direct framework adapters, plugin discovery, and suite/benchmark APIs; also had hardcoded light-theme colors and a stale proof band.

How

• Dark mode: ~370 hardcoded hexes mapped to CSS variables (the .dark palette existed; components weren't using it); @custom-variant dark added so Tailwind's dark: follows our class toggle; no-flash init script un-gated from /docs; shared ThemeToggle; solid buttons invert contrast correctly.
• Header: removed pill-morph + dead scroll state; brand left, nav + Docs + GitHub + toggle right-aligned; mobile overlay gains the toggle.
• Capabilities: 5 metrics as full-width spec rows (index | stat | text | viz) with hairline dividers — solves the odd-count orphan elegantly on desktop and mobile.
• New ScaleSection: scenario suites, A/B benchmark mode, custom-adapter registry — each with a mini code snippet.
• Content: hero terminal gained the RSR gate row; logo strip = deliberate 5/3 split with LangGraph/CrewAI icons; ProofBand updated to current reality (7 adapters, 5+ live frameworks, 300 tests); Product Hunt badge removed; ClosingCTA mentions benchmarking/recovery; smooth anchor scrolling with sticky-header offset.

Testing

• pnpm build compiles clean after every change; static pages generated
• Product untouched: make lint pass, 300 tests green
• Owner reviewed live at localhost:3000 across light/dark

Checklist

• Exactly one logical change in this PR (owner-directed site refresh)
• make lint passes
• uv run python -m pytest is green (300 passed)
• Docs updated if user-facing
• CHANGELOG entry - n/a (site only)
• Conventional title