Add sidebar for persistent information

[theantichris]

Description

Add a sidebar panel to display persistent information alongside the chat viewport.

Requirements

• Design sidebar content (model name, mode, token count, etc.)
• Implement layout that shares horizontal space with viewport
• Apply cyberpunk styling consistent with theme
• Consider: fixed vs responsive width, collapsible, left vs right placement
• Tests
• Documentation

Notes

Help/keybindings display is handled separately as a modal overlay (see #185).