fix: revert TypeScript 6.0 → 5.9 (typescript-eslint peer dep)

typescript-eslint@8.57.2 requires TypeScript <6.0.0. CI fails on
npm ci because strict peer dep resolution catches the conflict.
Local npm install succeeds because it uses cached lock file.

Author: austenstone

package-lock.json

@@ -49,7 +49,7 @@
     "jsdom": "^29.0.1",
     "npm-check-updates": "^19.6.6",
     "prettier": "^3.8.1",
-    "typescript": "~6.0.2",
+    "typescript": "~5.9.3",
     "typescript-eslint": "^8.57.2",
     "vite": "^8.0.3",
     "vitest": "^4.1.2"