* feat(ui): unify sidebar footer into full-width identity card
Footer becomes one card (avatar + name + chevron) mirroring the top agent
card; identity menu gains email header, Settings (with platform shortcut
hint), and Usage; Usage leaves default sidebar pins; settings shortcut now
also binds Ctrl+Shift+Comma; offline state lives in the card subtitle and
menu retry action and the card persists while disconnected.
* test(ui): align persisted-sidebar fallback default with usage removal