## ADDED Requirements

### Requirement: Theme toggle in sidebar
The UI shell SHALL include a theme toggle button in the sidebar. The button SHALL be positioned at the bottom of the sidebar, visually separated from the tool buttons.

#### Scenario: Toggle button renders
- **WHEN** the application loads
- **THEN** a theme toggle button is visible at the bottom of the sidebar

#### Scenario: Toggle button displays correct icon
- **WHEN** the current theme mode is "system"
- **THEN** the button shows a monitor icon (lucide-react `Monitor`)
- **WHEN** the current theme mode is "light"
- **THEN** the button shows a sun icon (lucide-react `Sun`)
- **WHEN** the current theme mode is "dark"
- **THEN** the button shows a moon icon (lucide-react `Moon`)

#### Scenario: Toggle button has tooltip
- **WHEN** user hovers over the theme toggle button
- **THEN** a tooltip displays the current mode name (e.g., "Theme: System", "Theme: Light", "Theme: Dark")