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

backend/src/app.ts

@@ -1,4 +1,4 @@
-import express from "express";
+import express, { Request, Response, NextFunction } from "express";
 import cors from "cors";
 import { config } from "./config";
 import healthRouter from "./routes/health";
@@ -13,5 +13,14 @@ export function createApp(): express.Express {
   app.use(healthRouter);
   app.use(racesRouter);
 
+  app.use((err: unknown, _req: Request, res: Response, _next: NextFunction) => {
+    if (err instanceof SyntaxError && "body" in err) {
+      res.status(400).json({ error: "validation_error", details: ["Invalid JSON in request body"] });
+      return;
+    }
+    console.error("[app] Unhandled error:", err);
+    res.status(500).json({ error: "unknown_error", details: ["Internal server error"] });
+  });
+
   return app;
 }