fix(onboarding): Commit the auto-detected platform to the host

The platform-features core only derived the displayed platform from
detection, committing to the host via onPlatformChange on explicit picks.
The onboarding step papers over this with a fallback commit in its
Continue handler, but the single-view project-creation flow has no
Continue boundary: accepting the detected platform left the wizard's
selectedPlatform unset, so the project name never defaulted and the
Create CTA stayed disabled with no way to proceed short of re-picking
the platform manually.

Commit the detected platform in the same once-per-repo effect that fires
the platform-selected analytics. The onboarding step's fallback commit
becomes a no-op and stays as belt and braces.

Author: jaydgoss

static/app/views/onboarding/components/scmPlatformFeaturesCore.tsx

@@ -161,11 +161,12 @@ export function ScmPlatformFeaturesCore({
 
   const currentPlatformName = getPlatformName(currentPlatformKey);
 
-  // Fire scm_platform_selected once per repo when detection auto-resolves a
-  // platform and the user hasn't explicitly chosen one. Otherwise a user who
-  // accepts the recommendation and clicks Continue never emits the event,
-  // leaving the funnel without a platform-selected step. The ref is re-armed
-  // on repo change above so a switch to a new repo fires the event again.
+  // Adopt the first detected platform once per repo when the user hasn't
+  // explicitly chosen one: commit it to the host so flows without a Continue
+  // boundary (single-view project creation) get a platform without an explicit
+  // pick, and fire scm_platform_selected so a user who just accepts the
+  // recommendation still emits a platform-selected funnel step. The ref is
+  // re-armed on repo change above so a switch to a new repo adopts again.
   useEffect(() => {
     if (
       autoDetectionTrackedRef.current ||
@@ -175,12 +176,19 @@ export function ScmPlatformFeaturesCore({
       return;
     }
     autoDetectionTrackedRef.current = true;
+    setPlatform(detectedPlatformKey);
     trackAnalytics(PLATFORM_SELECTED_EVENT[analyticsFlow], {
       organization,
       platform: detectedPlatformKey,
       source: 'detected',
     });
-  }, [detectedPlatformKey, selectedPlatform?.key, organization, analyticsFlow]);
+  }, [
+    detectedPlatformKey,
+    selectedPlatform?.key,
+    organization,
+    analyticsFlow,
+    setPlatform,
+  ]);
 
   // Wizard-driven platforms render an informational variant since the wizard CLI
   // owns product configuration and toggles aren't actionable.

static/app/views/onboarding/scmPlatformFeatures.spec.tsx

@@ -120,7 +120,7 @@ describe('ScmPlatformFeatures', () => {
     expect(within(radioGroup).getByText('Django')).toBeInTheDocument();
   });
 
-  it('auto-selects first detected platform', async () => {
+  it('auto-selects and commits first detected platform', async () => {
     MockApiClient.addMockResponse({
       url: `/organizations/${organization.slug}/repos/42/platforms/`,
       body: {
@@ -135,14 +135,15 @@ describe('ScmPlatformFeatures', () => {
       },
     });
 
-    render(
-      <ScmPlatformFeatures {...defaultProps({selectedRepository: mockRepository})} />,
-      {organization}
-    );
+    const props = defaultProps({selectedRepository: mockRepository});
+    render(<ScmPlatformFeatures {...props} />, {organization});
 
     expect(
       await screen.findByRole('heading', {level: 3, name: 'Available with Next.js'})
     ).toBeInTheDocument();
+    expect(props.onPlatformChange).toHaveBeenCalledWith(
+      expect.objectContaining({key: 'javascript-nextjs'})
+    );
   });
 
   describe('feature card variants', () => {