fix: add unrun devDependency for tsdown config loading

tsdown 0.22 resolves its config file through unrun; without it declared,
git-hosted installs (which run `pnpm install` + prepare inside the fetched
source) fail with "Failed to import module unrun". Declare it as a dev
dependency so the prepare step can load tsdown.config.ts.

package.json

@@ -167,6 +167,7 @@
     "react-dom": "18.2.0",
     "tsdown": "^0.22.2",
     "typescript": "^5.6.0",
+    "unrun": "^0.3.1",
     "vitest": "^4.1.8"
   }
 }