feat(gallery): add public templates gallery frontend (Tasks 4 & 5)

Add types, API client, page component, card components, detail modal,
and /templates route for the public templates gallery. Uses raw fetch()
for unauthenticated access, glass-card design system, and URL-synced
filters with debounced search.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/router.tsx

@@ -14,6 +14,7 @@ import {
 
 // Public pages
 const LandingPage = lazy(() => import('@/pages/LandingPage'))
+const PublicTemplatesPage = lazy(() => import('@/pages/PublicTemplatesPage'))
 const SharedSessionPage = lazy(() => import('@/pages/SharedSessionPage'))
 const SurveyPage = lazy(() => import('@/pages/SurveyPage'))
 const SurveyThankYouPage = lazy(() => import('@/pages/SurveyThankYouPage'))
@@ -92,6 +93,11 @@ export const router = sentryCreateBrowserRouter([
     element: page(LandingPage),
     errorElement: <RouteError />,
   },
+  {
+    path: '/templates',
+    element: page(PublicTemplatesPage),
+    errorElement: <RouteError />,
+  },
   {
     path: '/login',
     element: <LoginPage />,