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/styles/global.css

@@ -64,7 +64,9 @@ a {
   outline: none;
 }
 
-.app-shell__link--active {
+.app-shell__link--active,
+.app-shell__link--active:hover,
+.app-shell__link--active:focus-visible {
   color: var(--color-surface);
   background: var(--color-accent);
 }