Dark/light theme support

[GordonBeeming]

Description

Implement theme support with system detection and manual override.

Acceptance Criteria

• Detect OS theme preference (dark/light)
• Manual toggle: Settings → Theme: System / Dark / Light
• Monaco editor theme synced (vs-dark / vs-light)
• Results grid, tree view, dialogs all respect theme
• Theme preference persisted
• CSS variables for all colors (easy to extend with custom themes later)
• Tailwind CSS dark mode variant