Give the frontend's check scripts something that runs them

Eight `*.check.ts` scripts existed and nothing ran any of them, so a
helper's arithmetic was only checked by hand. `make test-checks` runs
every one and `make test` now includes it — they need no stack, so they
sit between the backend suite and Playwright.

frontend/package.json

@@ -12,7 +12,8 @@
     "preview": "vite preview",
     "generate-client": "openapi-ts",
     "test": "bunx playwright test",
-    "test:ui": "bunx playwright test --ui"
+    "test:ui": "bunx playwright test --ui",
+    "check": "for f in $(find src -name '*.check.ts' | sort); do echo \"  → $f\"; bun run \"$f\" || exit 1; done"
   },
   "dependencies": {
     "@hookform/resolvers": "^5.2.2",