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/package.json

@@ -8,7 +8,7 @@
     "start": "node dist/index.js",
     "db:migrate": "ts-node src/migrate.ts",
     "seed": "ts-node src/seed.ts",
-    "test": "CALENDAR_RUN_MOCK_DB=1 tsx --test test/app.test.ts"
+    "test": "cross-env CALENDAR_RUN_MOCK_DB=1 tsx --test test/app.test.ts"
   },
   "dependencies": {
     "cors": "^2.8.5",
@@ -23,6 +23,7 @@
     "@types/node": "^22.12.0",
     "@types/pg": "^8.11.10",
     "@types/supertest": "^6.0.2",
+    "cross-env": "^10.1.0",
     "supertest": "^7.0.0",
     "ts-node": "^10.9.2",
     "tsx": "^4.19.2",