fix: icon permissions and GTK single instance

- Use GTK default flags for single instance
- Fix icon path to absolute /usr/share/icons
- Add postinstall chmod 644 for icon
- Remove Dart-level single instance code

linux/packaging/deb/make_config.yaml

@@ -11,6 +11,7 @@ essential: false
 icon: ./logo/ic_launcher_playstore.png
 
 postinstall_scripts:
+  - chmod 644 /usr/share/icons/hicolor/256x256/apps/umbrix.png
   - echo "Installed Umbrix VPN"
 postuninstall_scripts:
   - echo "Uninstalled Umbrix"