Remove MSW handlers and update demo credentials for improved standardization

- Deleted `admin.ts`, `auth.ts`, and `users.ts` MSW handler files to streamline demo mode setup.
- Updated demo credentials logic in `DemoCredentialsModal` and `DemoModeBanner` for stronger password requirements (≥12 characters).
- Refined documentation in `CLAUDE.md` to align with new credential standards and auto-generated MSW workflows.

backend/app/core/demo_data.json

@@ -32,6 +32,16 @@
         }
     ],
     "users": [
+        {
+            "email": "demo@example.com",
+            "password": "DemoPass1234!",
+            "first_name": "Demo",
+            "last_name": "User",
+            "is_superuser": false,
+            "organization_slug": "acme-corp",
+            "role": "member",
+            "is_active": true
+        },
         {
             "email": "alice@acme.com",
             "password": "Demo123!",