chore: migrate from npm to pnpm

- Replace package-lock.json with pnpm-lock.yaml (pnpm 11.7.0)
- Add packageManager field to package.json
- Add pnpm-workspace.yaml with approved build scripts (esbuild, unrs-resolver)
- Verified: 25/25 tests pass, lint clean, static build OK

package.json

@@ -30,5 +30,6 @@
     "tailwindcss": "^3.4.1",
     "typescript": "^5",
     "vitest": "^1.6.1"
-  }
+  },
+  "packageManager": "pnpm@11.7.0"
 }