fix(pnpm): pin packageManager to pnpm@10.28.0

Prevents corepack from downloading pnpm 11 in Docker, which has a new
security policy (ERR_PNPM_IGNORED_BUILDS) incompatible with how this
project's builds are configured. Locks to the version used locally.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

package.json

@@ -6,6 +6,7 @@
     "name": "Valknar",
     "email": "valknar@pivoine.art"
   },
+  "packageManager": "pnpm@10.28.0",
   "scripts": {
     "dev": "next dev --turbopack",
     "build": "next build",