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/tokens.css

@@ -16,6 +16,7 @@
   --font-size-caption: 0.875rem;
   --line-height-base: 1.5;
 
+  --space-1: 0.25rem;
   --space-2: 0.5rem;
   --space-3: 0.75rem;
   --space-4: 1rem;