Adopt new nav layouts and pagination across refrakt.md

Acceptance Criteria

Header — menubar

• site/content/_layout.md header region replaces the plain markdown link row with {% nav layout="menubar" %}
• Header nav defines top-level items (logo / Docs / Blog / GitHub) and at least two grouped categories — proposed: Product (pricing, features, runes catalogue, plugins) and Resources (about, blog, changelog, plan)
• Exact group composition is decided as part of this work item; capture the chosen grouping in the resolution notes for future reference
• GitHub / external-link items keep working (verify the existing {% icon name="github" /%} pattern survives the migration)

Footer — columns

• site/content/_layout.md footer region uses {% nav layout="columns" %} (or adds a footer region if one doesn't exist yet)
• Footer defines at least three columns (proposal: Product, Resources, Legal); column composition decided during implementation and captured in resolution notes
• Copyright / "small print" row stays as plain markdown beneath the nav (out of scope for a dedicated rune in this milestone)

Sidebars — collapsible

• site/content/docs/_layout.md switches its sidebar {% nav %} to {% nav collapsible %}
• site/content/runes/_layout.md switches its sidebar {% nav %} to {% nav collapsible %}
• Verify on dev server: navigating to a page inside a group auto-expands only that group; other groups stay collapsed
• No defaultOpen overrides — rely entirely on URL-driven auto-open, since that's the demo case

Section landing — cards

• At least one section landing page adopts {% nav layout="cards" %}. Candidates: site/content/runes/rune-catalog.md, site/content/docs/index.md, site/content/docs/plugins/index.md. Pick one and capture the choice in the resolution
• If the chosen page already has a hand-authored card grid (e.g. via bento or feature), replace it with the new nav-driven cards so the enrichment path is exercised
• For each child page referenced by the cards nav, verify the page has title and description frontmatter, and add icon: <name> frontmatter where one would help — pick a sensible icon from the Lumina set

Pagination — docs and runes layouts

• site/content/docs/_layout.md adds {% pagination auto /%} (placed below the main {% region name="content" %} or in a dedicated pagination region the layout defines)
• site/content/runes/_layout.md adds {% pagination auto /%}
• Verify on dev server: navigating through docs / rune pages in order, prev / next links appear at the expected positions; first and last pages omit the appropriate side
• Verify ordering matches the declared nav order in each layout's sidebar — not directory order — since both layouts have an explicit nav order that should win

Cross-cutting

• Dev server boots cleanly; no console warnings about the new runes
• site && npm run build succeeds
• At least one screen capture (or video) of the header dropdowns and the collapsible sidebar attached to the PR description for review