feat(settings): Integration & polish

[tomymaritano]

Summary

Final integration and polish for the settings window.

Tasks

• Add keyboard shortcut (Cmd+,) to open settings
• Add settings button in sidebar header
• Persist window position/size
• Settings search (optional)

Dependencies

• Depends on feat(settings): Schema foundation + Editor/General settings #63 (Schema foundation)
• Depends on feat(settings): Appearance section + theme switching #64 (Appearance)