fix: use --workspaces (plural) in install:contracts script

--workspace=packages targets a workspace named "packages" which doesn't
exist. The config is "packages/*" so the actual workspaces are the
individual subdirectories. --workspaces targets all of them.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: oceans404

package.json

@@ -7,7 +7,7 @@
     "dev": "npm start",
     "start": "concurrently --kill-others-on-fail --names stellar,vite -c gray,green --pad-prefix \"stellar scaffold watch --build-clients\" \"vite\"",
     "build": "tsc -b && vite build",
-    "install:contracts": "npm install --workspace=packages && npm run build --workspace=packages",
+    "install:contracts": "npm install --workspaces && npm run build --workspaces",
     "preview": "vite preview",
     "lint": "eslint .",
     "format": "prettier . --write",