fix(deploy): pin packageManager to pnpm@11.1.2 so Coolify uses the right pnpm

Last build crashed with ERR_PNPM_LOCKFILE_CONFIG_MISMATCH on
patchedDependencies — Nixpacks bundles pnpm 9.15.9 by default, but our
lockfile is in pnpm 11's settings/patchedDependencies format. Adding the
'packageManager' field tells corepack-aware tooling (Nixpacks included)
to install pnpm 11.1.2 in the build container, matching what we use
locally. After this, 'pnpm i --frozen-lockfile' reads the same lockfile
format the project was developed with and the patched dependency
declaration lines up.

package.json

@@ -2,6 +2,7 @@
   "name": "lab",
   "version": "0.1.0",
   "private": true,
+  "packageManager": "pnpm@11.1.2",
   "scripts": {
     "dev": "next dev",
     "build": "next build",