Lens color paints the whole shell, not just the center column

The earlier implementation only colored the center column, leaving the rails as cream strips around (e.g.) a stone-50 Calm feed. That looked like a UI bug, not a mood. A lens isn't a cosmetic tweak — it's the user telling the substrate what kind of moment they're in. The whole frame should reflect that.

Implementation: pathname is mirrored into a request header by the proxy (src/proxy.ts), the layout reads it server-side via headers() to set the initial CSS variable, and a client component (lens-bg-sync.tsx) updates the variable on soft navigations because Next.js App Router preserves layouts across same-segment route changes.