feat(10-01): update App.test.tsx and add step descriptions to all 4 steps

- Update App.test.tsx: add 'shows intro section on initial render' test
- Update App.test.tsx: fix 'renders BackendSelectionStep' — click Get Started first
- Add fireEvent import for click simulation
- BackendSelectionStep: add description below h2 (text-sm text-on-surface-variant)
- RemoteConfigStep: add description below h2
- DeploymentStep: add description below h2
- ReviewStep: add h2 heading + description paragraph (step was missing h2)
- All descriptions use text-sm text-on-surface-variant mt-1 mb-4

src/components/wizard/ReviewStep.tsx

@@ -72,6 +72,12 @@ export function ReviewStep() {
 
   return (
     <div>
+      <h2>Step 4: Review &amp; Download</h2>
+      <p className="text-sm text-on-surface-variant mt-1 mb-4">
+        Review your generated files and download them. Acknowledge the security notice before
+        copying or downloading credentials.
+      </p>
+
       {/* SECU-02: client-side only notice */}
       <p className="text-sm text-success bg-success/10 rounded p-3 mb-6">
         No data is sent to any server. All file generation happens in your browser.