Add CSS lint tasks, ignore CSS; relax test stub type

Add CSS lint steps to frontend tasks: run lint:css:fix during format and lint:css:check during build in .taskfiles/frontend.yml. Exclude **/*.css from Prettier in frontend/.prettierignore because CSS is formatted/validated with Stylelint. Adjust test helper type in frontend/editor/src/core/tests/helpers/api-stubs.ts to allow the nested MockAppApiOptions object to be null, making test stubs more flexible.

frontend/.prettierignore

@@ -6,6 +6,8 @@ editor/dist/
 editor/src-tauri/**/target/
 editor/src-tauri/gen/
 node_modules/
+# CSS is formatted and validated separately with Stylelint.
+**/*.css
 editor/public/vendor/
 # Auto-generated by MSW (`msw init`); regenerated verbatim, not hand-formatted.
 editor/public/mockServiceWorker.js

frontend/editor/src/core/tests/helpers/api-stubs.ts

@@ -126,7 +126,7 @@ export interface MockAppApiOptions {
     username?: string;
     email?: string;
     roles?: string[];
-  };
+  } | null;
   /** Languages advertised by `/config/app-config`. */
   languages?: string[];
   /** Default locale. */