fix: phase 1 bugs — CSS tokens, pluralization, error handling, cross-platform tests

- Add missing --space-1 CSS token used by filter and detail components

- Fix active nav link losing styles on hover (CSS specificity)

- Correct Russian day pluralization (21 день, 22 дня, 25 дней)

- Show filter error banner even when stale race data is present

- Add cross-env for Windows-compatible npm test

- Add global JSON error handler in Express for malformed request bodies

- Replace stateless mock DB with in-memory store for correct DELETE/UPDATE behavior

Made-with: Cursor

frontend/src/pages/RacesPage.tsx

@@ -145,6 +145,12 @@ export function RacesPage(): JSX.Element {
       <h1 className="page__title">Календарь стартов</h1>
       <p className="page__subtitle">Будущие и прошедшие старты в одном месте.</p>
 
+      {errorMessage && !isLoading ? (
+        <p className="page__subtitle page__subtitle--error" role="alert" style={{ marginTop: "var(--space-4)" }}>
+          {errorMessage}
+        </p>
+      ) : null}
+
       <div className="races-filter" role="search" aria-label="Фильтр по дате">
         <label className="races-filter__field">
           <span className="races-filter__label">Год</span>