Add a persistent global top navigation bar (home + docs)

[nficano]

Summary

The site has no global navigation. Docs expose only a left sidebar (the ARCP wordmark is buried in its top-left corner); the home header is a wordmark + three non-interactive tag spans. Global destinations — Docs, Spec, GitHub, search, theme toggle — have nowhere to live, and the home and docs layouts don't share a shell.

Location(s)

• Site-wide (home + all docs) — app/components/HomeHeader.vue, app/layouts/docs.vue.

Severity

P2 — IA / cohesion gap for a public launch.

Evidence

• HomeHeader.vue: wordmark (/) + v-for over tags rendered as plain <span> (not links).
• docs.vue: left sidebar only, no top bar.
• No GitHub link, search, or theme toggle anywhere in either layout.

Suggested fix

Add a shared top navbar across both layouts:

• ARCP wordmark · primary links (Docs / Spec / GitHub) · language switcher · search · theme toggle.
• Move global concerns out of the sidebar so the sidebar is purely page-nav.

This resolves the navigation half of #16 and contributes to the home↔docs cohesion raised in #11.

Acceptance criteria

• Persistent top nav on home and docs linking to docs, spec, and GitHub
• Language switcher (and search/theme controls) live in the top bar
• Reaching any SDK's docs from / is one click