fix: restore packageManager field for Cloudflare Workers Builds (#104)

PR #101 replaced the corepack 'packageManager' string with the newer
'devEngines' object. Cloudflare Workers Builds (the production deploy
pipeline for rustfs.com) resolves the pnpm version via the
'packageManager' field; without it every build since 9115f61 has
failed, so merged changes (including #103) never went live.

Restore the field pinned to pnpm@12.3.4, matching the version #101
intended, while keeping the devEngines block.

Author: majinghe

package.json

@@ -3,6 +3,7 @@
   "version": "0.1.0",
   "private": true,
   "type": "module",
+  "packageManager": "pnpm@12.3.4",
   "scripts": {
     "dev": "next dev",
     "build": "next build",