Update missing direction creation (#1146)

Update the doc for missing userData for the database.
    
<!-- This is an auto-generated description by cubic. -->
---

## Summary by cubic
Add a setup step in CONTRIBUTING.md to create the userData directory
required by the database. This prevents migration commands from failing
on fresh installs.

<!-- End of auto-generated description by cubic. -->

CONTRIBUTING.md

@@ -20,9 +20,23 @@ Dyad is an Electron app.
 npm install
 ```
 
+**Create the userData directory (required for database)**
+
+```sh
+# Unix/macOS/Linux:
+mkdir -p userData
+
+# Windows PowerShell (run only if folder doesn't exist):
+mkdir userData
+
+# Windows Command Prompt (run only if folder doesn't exist):
+md userData
+```
+
 **Apply migrations:**
 
 ```sh
+# Generate and apply database migrations
 npm run db:generate
 npm run db:push
 ```