docs(onboarding): add environment preflight screenshot to quickstart (#1296)

Show the first-run environment preflight step in the quickstart so the
setup walkthrough is fully illustrated between the Cold start card and the
dashboard. The screenshot renders the six checks (Docker engine, Compose,
compose directory, path mapping, TLS, disk) with their results, the Re-run
control, and the Enter Sencho action.

docs/getting-started/quickstart.mdx

@@ -89,6 +89,10 @@ Pick a username, choose a password, confirm it, and click **Initialize console**
 
 Sencho then runs a short **environment preflight**: it confirms the Docker engine and Compose plugin are reachable, the compose directory is writable and mounted at a matching host path, the dashboard is behind TLS, and there is disk headroom. Anything that needs attention shows an inline fix. The checks never block you, so click **Enter Sencho** to continue; you can re-run them anytime from **Settings · Recovery**.
 
+<Frame>
+  <img src="/images/quickstart/setup-environment.png" alt="Sencho first-boot environment preflight with check rows for Docker engine, Docker Compose, Compose directory, path mapping, TLS, and disk space, each showing an OK or warning result, a Re-run button, and an Enter Sencho button" />
+</Frame>
+
 <Note>
   The Cold start card only appears the first time you open Sencho. Once the admin account exists, every subsequent visit goes to the regular sign-in screen.
 </Note>