fix(docs-site): expose sections in mobile menu

[vyctorbrzezowski]

Summary

• add a compact top-level section switcher to the proposed mobile drawer
• keep the active section's page navigation directly below it
• simplify the proposed mobile header while preserving language and theme controls
• cover drawer geometry, section state, focus, and responsive header behavior

Context

Thanks to @bdjben for surfacing and documenting the mobile navigation discovery gap in openclaw/openclaw#100908. This PR builds on that work by exposing the same top-level sections in the site shell on every docs page.

Proposed UI

Verification

• npm test
• npm run docs:build:shell
• npm run docs:smoke:shell
• node --check scripts/docs-site/build.mjs
• node --check scripts/docs-site/assets.mjs
• node --check scripts/docs-site/smoke.mjs
• node --check scripts/docs-site/visual-smoke.mjs
• Playwright checks at 390 px and 603 px for section navigation, drawer closure, header alignment, and horizontal overflow

[vyctorbrzezowski]

Merged via squash.

• Reviewed head SHA: 47c140e98f80bc61631ff09cc970fe7eda346cc8
• Landed commit: c22db64fb16b9706b81350480ca160380c1dff2b
• Exact-head validation: Docs Code CI run 29172321641