Social presence and editor polish

[jonesd]

Scope

Split out from #35 (Phases 4-5). Core collaborative editing (Phases 1-3) is complete.

Phase 4: Social presence

• Global presence (see who is online across all works)
• Activity feed (work created, edited, shared)
• Join a friend's active work

Phase 5: Polish

• Undo/redo awareness (remote undo visible to collaborators)
• Mobile responsive layout
• Keyboard shortcuts

Context

Lower priority than core features (#18 compound documents, #12 edition structural ops). File when collaborative editing sees real multi-user usage and the need becomes clear.

Ref: #35