feat: admin survey responses page with expandable detail and CSV export

- Backend: GET /admin/survey-responses (list with stats, invite join)
- Backend: GET /admin/survey-responses/export (CSV download)
- Frontend: SurveyResponsesPage with expandable row detail
- Two-column Q&A grid with typed answer rendering (chips, ranked lists, quote blocks)
- Stats cards (total responses, this week)
- CSV export button with blob download
- Sidebar nav + route wiring
- Also: updated Q14 from product domain ranking to diagnostic prioritization

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/router.tsx

@@ -52,6 +52,7 @@ const AdminFeatureFlagsPage = lazy(() => import('@/pages/admin/FeatureFlagsPage'
 const AdminSettingsPage = lazy(() => import('@/pages/admin/SettingsPage'))
 const AdminGlobalCategoriesPage = lazy(() => import('@/pages/admin/GlobalCategoriesPage'))
 const AdminSurveyInvitesPage = lazy(() => import('@/pages/admin/SurveyInvitesPage'))
+const AdminSurveyResponsesPage = lazy(() => import('@/pages/admin/SurveyResponsesPage'))
 
 // Account pages
 const AccountLayout = lazy(() => import('@/components/account/AccountLayout'))
@@ -403,6 +404,14 @@ export const router = createBrowserRouter([
               </Suspense>
             ),
           },
+          {
+            path: 'survey-responses',
+            element: (
+              <Suspense fallback={<PageLoader />}>
+                <AdminSurveyResponsesPage />
+              </Suspense>
+            ),
+          },
         ],
       },
       // Account routes