test(frontend): verify global stylesheet @import graph after vitest

Add scripts/check-css-import-graph.mjs and run it from npm test and
test:coverage so missing relative CSS imports fail CI like Vite/postcss.
Document the step in src/test/README.md; trigger frontend workflow when
the script changes.

scripts/check-css-import-graph.mjs

@@ -0,0 +1,63 @@
+#!/usr/bin/env node
+/**
+ * Walk the same root CSS entrypoints as src/main.tsx and ensure every
+ * relative @import target exists on disk. Catches postcss-import ENOENT
+ * that Vitest does not surface (CSS is not fully resolved like vite build).
+ */
+import fs from 'node:fs';
+import path from 'node:path';
+import { fileURLToPath } from 'node:url';
+
+const __dirname = path.dirname(fileURLToPath(import.meta.url));
+const repoRoot = path.resolve(__dirname, '..');
+const stylesRoot = path.join(repoRoot, 'src', 'styles');
+
+const ENTRY_RELS = [
+  'themes/index.css',
+  'layout/index.css',
+  'components/index.css',
+  'tracks/index.css',
+];
+
+/** Quoted path in @import '...' or "..." (postcss-import style in this repo). */
+const IMPORT_RE = /@import\s+(['"])([^'"]+)\1/g;
+
+function isFilesystemImport(spec) {
+  if (spec.startsWith('http://') || spec.startsWith('https://') || spec.startsWith('data:')) {
+    return false;
+  }
+  if (spec.startsWith('@')) return false;
+  return spec.startsWith('./') || spec.startsWith('../');
+}
+
+function walk(absPath, visited, missing) {
+  const key = path.resolve(absPath);
+  if (visited.has(key)) return;
+  if (!fs.existsSync(key)) {
+    missing.add(path.relative(repoRoot, key));
+    visited.add(key);
+    return;
+  }
+  visited.add(key);
+  const text = fs.readFileSync(key, 'utf8');
+  let m;
+  IMPORT_RE.lastIndex = 0;
+  while ((m = IMPORT_RE.exec(text)) !== null) {
+    const spec = m[2];
+    if (!isFilesystemImport(spec)) continue;
+    const next = path.resolve(path.dirname(key), spec);
+    walk(next, visited, missing);
+  }
+}
+
+const visited = new Set();
+const missing = new Set();
+for (const rel of ENTRY_RELS) {
+  walk(path.join(stylesRoot, rel), visited, missing);
+}
+
+if (missing.size > 0) {
+  const list = [...missing].sort().join('\n  ');
+  console.error(`check-css-import-graph: missing @import target(s):\n  ${list}`);
+  process.exit(1);
+}