Major UI overhaul based on 4-reviewer design council. Addresses all 5 key findings.
Changes:
- Two-row header: brand + model selector (top), session + tokens (below)
- Empty state with prompt chips for new sessions
- Toolbar collapsed: 3 items + overflow menu
- Single gold accent (dropped orange)
- Full VS Code theme adaptation via --vscode-* CSS variables
- User bubbles now use theme colors with gold accent border
See CHANGELOG.md for details.