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.

src/test/README.md

@@ -33,8 +33,21 @@ src/test/
 npm test                       # one-shot run
 npm run test:watch             # watch mode
 npm run test:coverage          # with v8 coverage → ./coverage/
+npm run check:css-imports      # only the global stylesheet @import graph (see below)
 ```
 
+## CSS `@import` graph
+
+Vitest does not load the full global CSS bundle from `main.tsx`, so a broken
+relative `@import` under `src/styles/**` can slip past the suite until Vite
+runs (`ENOENT` from postcss-import).
+
+After **`vitest run`**, **`npm test`** and **`npm run test:coverage`** run
+**`npm run check:css-imports`**, which executes **`scripts/check-css-import-graph.mjs`**
+and walks the same four root stylesheets as **`src/main.tsx`**, resolving
+only filesystem-relative imports (`./…`, `../…`). Package imports such as
+`@fontsource/...` are ignored.
+
 ## Where tests go
 
 - **Co-located with the unit under test**: `Foo.tsx` → `Foo.test.tsx`,