feat: add AI chat builder frontend — types, API client, store, components, page, routing

- TypeScript types for chat session, messages, and responses
- API client module with all 6 endpoints
- Zustand store with session management, message sending, tree generation, import, resume
- 7 chat components: ChatMessage, ChatInput, ChatPanel, PhaseIndicator, ChatToolbar, EmptyPreview, StaticTreePreview
- AIChatBuilderPage with split-panel layout (60% chat / 40% preview)
- Route at /ai/chat with lazy loading
- "Build with AI" button on TreeLibraryPage
- Session resume via URL search params

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

frontend/src/router.tsx

@@ -33,6 +33,7 @@ const TeamAnalyticsPage = lazy(() => import('@/pages/TeamAnalyticsPage'))
 const MyAnalyticsPage = lazy(() => import('@/pages/MyAnalyticsPage'))
 const FeedbackPage = lazy(() => import('@/pages/FeedbackPage'))
 const StepLibraryPage = lazy(() => import('@/pages/StepLibraryPage'))
+const AIChatBuilderPage = lazy(() => import('@/pages/AIChatBuilderPage'))
 const AccountSettingsPage = lazy(() => import('@/pages/AccountSettingsPage'))
 // Admin pages
 const AdminLayout = lazy(() => import('@/components/admin/AdminLayout'))
@@ -253,6 +254,14 @@ export const router = createBrowserRouter([
           </Suspense>
         ),
       },
+      {
+        path: 'ai/chat',
+        element: (
+          <Suspense fallback={<PageLoader />}>
+            <AIChatBuilderPage />
+          </Suspense>
+        ),
+      },
       // Admin routes
       {
         path: 'admin',