The dashboard's sidebar shell: a collapsible rail on desktop (toggle button, drag edge, Cmd/Ctrl+B) that remembers its state across reloads, becomes a slide-in drawer on mobile, and provides the header/content/footer/menu building blocks pages fill in — with tooltips standing in for labels when collapsed to icons.

## Before writing SPEC.md files

Read https://raw.githubusercontent.com/brillout/sdd/refs/heads/main/sdd.md
