Shells

The three ground types of an app shell (docs/shells.md). Each demo is a mock app with its own header; try them at phone width.

• Content-firstA regular app: sidebar + main content; the phone shows the content, the sidebar slides in.
• List-firstNotes: the phone shows the list first, an item replaces it; desktop shows both.
• Three-paneMail: folders, the list, the opened item with its actions; desktop shows all three.