feat: create dedicated desktop dock icon and update nix build to use it

- Add app/sovran_systemsos_web/static/sovran-hub-icon.svg: a new square
  256x256 app icon for the GNOME dock/dash. Uses the Sovran brand dark
  green (#0d3320) rounded-rectangle background, concentric arc rings in
  brand greens (#1C9954, #077233), and a white bold "S" letterform
  centered — visible at small sizes on both light and dark panels.
- Update modules/core/sovran-hub.nix line 266 to copy the new icon file
  to the hicolor icon path instead of reusing logo-light.svg.
- logo-light.svg is left untouched; it continues to serve the Hub web UI.

Agent-Logs-Url: https://github.com/naturallaw777/staging_alpha/sessions/e3f466ae-eee1-4ba8-b93c-00fe04c7054d

Co-authored-by: naturallaw777 <99053422+naturallaw777@users.noreply.github.com>

modules/core/sovran-hub.nix

@@ -263,7 +263,7 @@ let
       cp icons/* $out/share/sovran-hub/icons/ 2>/dev/null || true
 
       install -d $out/share/icons/hicolor/scalable/apps
-      cp sovran_systemsos_web/static/logo-light.svg $out/share/icons/hicolor/scalable/apps/sovran-hub.svg
+      cp sovran_systemsos_web/static/sovran-hub-icon.svg $out/share/icons/hicolor/scalable/apps/sovran-hub.svg
 
       install -d $out/share/applications
       cat > $out/share/applications/sovran-hub.desktop <<DESKTOP