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

pyproject.toml

@@ -20,6 +20,8 @@ classifiers = [
 dependencies = [
     "fabric",
     "httpx>=0.28.1",
+    "keyring>=23.0.0",
+    "keyrings-alt>=5.0.2",
     "psutil>=7.2.2",
     "python-pam>=2.0.2",
     "tomlkit>=0.15.0",