feat: add sysauth service for Polkit authentication

- Add SysauthService with D-Bus agent registration
- Add AuthDialog with centered overlay password prompt
- Add password utils (PAM verification, keyrings.alt caching)
- Integrate sysauth into main app initialization
- Update WiFi dialog to use sysauth for password prompts
- Add sysauth.css with dialog styling
- Remove standalone WiFi password dialog

src/services/__init__.py

@@ -20,5 +20,6 @@ from .network import (  # noqa: F401
     Wifi,
 )
 from .numlock import NumLock  # noqa: F401
+from .sysauth import SysauthService, get_sysauth_service  # noqa: F401
 from .todo import TodoService  # noqa: F401
 from .wallpaper import WallpaperService, create_thumbnail  # noqa: F401