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/api/aiChat.ts

@@ -0,0 +1,44 @@
+import { apiClient } from './client'
+import type {
+  AIChatStartResponse,
+  AIChatMessageResponse,
+  AIChatSessionResponse,
+  AIChatGenerateResponse,
+  AIChatImportResponse,
+} from '@/types'
+
+export const aiChatApi = {
+  startSession: async (flowType: 'troubleshooting' | 'procedural'): Promise<AIChatStartResponse> => {
+    const { data } = await apiClient.post('/ai/chat/sessions', { flow_type: flowType })
+    return data
+  },
+
+  sendMessage: async (sessionId: string, content: string): Promise<AIChatMessageResponse> => {
+    const { data } = await apiClient.post(`/ai/chat/sessions/${sessionId}/messages`, { content })
+    return data
+  },
+
+  getSession: async (sessionId: string): Promise<AIChatSessionResponse> => {
+    const { data } = await apiClient.get(`/ai/chat/sessions/${sessionId}`)
+    return data
+  },
+
+  generateTree: async (sessionId: string): Promise<AIChatGenerateResponse> => {
+    const { data } = await apiClient.post(`/ai/chat/sessions/${sessionId}/generate`)
+    return data
+  },
+
+  importTree: async (
+    sessionId: string,
+    params?: { name?: string; description?: string; category_id?: string; tags?: string[] }
+  ): Promise<AIChatImportResponse> => {
+    const { data } = await apiClient.post(`/ai/chat/sessions/${sessionId}/import`, params || {})
+    return data
+  },
+
+  abandonSession: async (sessionId: string): Promise<void> => {
+    await apiClient.delete(`/ai/chat/sessions/${sessionId}`)
+  },
+}
+
+export default aiChatApi

frontend/src/api/index.ts

@@ -17,3 +17,4 @@ export { targetListsApi } from './targetLists'
 export { maintenanceSchedulesApi, batchLaunchApi } from './maintenanceSchedules'
 export { default as feedbackApi } from './feedback'
 export { default as aiBuilderApi } from './aiBuilder'
+export { default as aiChatApi } from './aiChat'