Workspaces, and a rebuilt layout

Brains now live inside workspaces, each with its own members and overview page, so a team that works on several things at once can keep them apart. Every brain got a detail page with sharing, renaming, and settings in one place.

The app layout was rebuilt around this — a real sidebar with a single source of truth for what's highlighted, billing moved into the org selector, and personal and team content separated instead of interleaved.