feat: System appearance sync

[ikorich]

Summary

Auto-switch theme mode based on macOS system appearance.

Motivation

Seamless integration with macOS dark/light mode preferences.

Implementation

• Add "System" option to theme mode
• Monitor NSAppearance changes
• Auto-switch theme when system changes

Files to Modify

• SettingsTypes.swift - Add system option to ThemeMode
• ToolBarView.swift - Update mode toggle
• CodeEditorView.swift - React to system appearance

Acceptance Criteria

• "System" option in theme mode
• Theme auto-switches with macOS
• Manual override still works
• Export uses current theme

Story Points

2 points