feat(desktop): add native window controls toggle

[dmasiero]

Summary

• add a persistent Appearance setting for native window controls
• apply the preference immediately and during app startup
• grant the required Tauri window capability
• preserve Buzz's custom drag region when decorations are hidden

Test plan

• toggle native controls off and confirm the title bar disappears
• confirm the window remains draggable and double-click maximize still works
• toggle controls back on and confirm the title bar returns
• restart Buzz and confirm the preference persists

[dmasiero]

Native controls enabled

Native controls disabled