fix(stackblitz): bootstrap pnpm 10 in WebContainer start command (#464)

StackBlitz WebContainers ship a pinned pnpm 8.15.6, which fails the
repo's `engines.pnpm >=10.0.0` gate under `engine-strict=true` and
cannot read the lockfileVersion 9.0 pnpm-lock.yaml. The "Open in
StackBlitz" flow therefore died on `pnpm install` with
ERR_PNPM_UNSUPPORTED_ENGINE.

Prepend `npm i -g pnpm@10.33.0` (matching the `packageManager` field)
to the startCommand so the WebContainer installs a compatible pnpm
before install/dev. Keeps engine-strict and the Node 22 / pnpm 10
requirements intact for real dev + CI. Config-only; does not affect
the publishable dist/objectstack.json.

Co-authored-by: Jack Zhuang <277994282+os-zhuang@users.noreply.github.com>
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

Author: os-zhuang

.stackblitzrc

@@ -1,6 +1,6 @@
 {
   "installDependencies": false,
-  "startCommand": "pnpm install && pnpm dev",
+  "startCommand": "npm i -g pnpm@10.33.0 && pnpm install && pnpm dev",
   "env": {
     "OS_DATABASE_URL": ".objectstack/data/hotcrm.wasm.db",
     "OS_DATABASE_DRIVER": "sqlite-wasm"