Add accessible mobile navigation sidecar

[OwenMcGirr]

Summary

• replace the wrapping mobile header navigation with an inline-end modal sidecar at widths up to 800px
• use native <dialog> behavior for modality, focus containment, Escape handling, and inert background content
• animate the panel with Motion while respecting reduced-motion preferences and logical RTL direction
• preserve desktop navigation and keep Clerk's portalled account control in the compact header
• cover signed-in, signed-out, loading, unconfigured, navigation, dismissal, breakpoint, scroll-lock, and accessibility behavior

Test plan

• pnpm --filter @opensymbols/site test -- src/components/layout.test.tsx
• pnpm typecheck
• pnpm lint (passes with 3 pre-existing Fast Refresh warnings)
• pnpm test
• pnpm build
• docker compose config --quiet
• git diff --check
• manually exercised 320px, 390px, and 801px viewports, initial focus, Escape, route dismissal, live breakpoint changes, reduced motion, scroll restoration, and horizontal reflow

Bundle impact

Motion is loaded through LazyMotion and m, avoiding Vite's 500 kB chunk warning. The site bundle changes from 406.17 kB / 121.18 kB gzip to 484.38 kB / 148.34 kB gzip: +78.21 kB minified / +27.16 kB gzip.

Scope and risk

• no server, API, route, or authentication contract changes
• PR Add provider-neutral Meilisearch discovery #54 remains open and unchanged
• native dialog support is required; current supported browsers provide it

Closes #55