refactor: compose UI shell with explicit application operations and component owners

src/testSupport/readStylesheet.mjs

@@ -0,0 +1,11 @@
+/** Resolve the actual ordered local CSS imports used by the application entry. */
+import { readFileSync } from 'node:fs';
+import path from 'node:path';
+import { fileURLToPath } from 'node:url';
+export function readStylesheet(file, ancestors = new Set()) {
+  const resolved = path.resolve(file instanceof URL ? fileURLToPath(file) : file);
+  if (ancestors.has(resolved)) throw new Error('Circular stylesheet import');
+  const next = new Set(ancestors).add(resolved);
+  return readFileSync(resolved, 'utf8').replace(/@import ['"]([^'"]+)['"];\s*/g,
+    (_, imported) => readStylesheet(path.resolve(path.dirname(resolved), imported), next));
+}