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/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'