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/types/ai-chat.ts

@@ -0,0 +1,43 @@
+export type InterviewPhase = 'scoping' | 'discovery' | 'enrichment' | 'review' | 'generation'
+
+export interface ChatMessage {
+  role: 'user' | 'assistant'
+  content: string
+  timestamp: string
+}
+
+export interface AIChatStartResponse {
+  session_id: string
+  greeting: string
+  current_phase: InterviewPhase
+}
+
+export interface AIChatMessageResponse {
+  content: string
+  current_phase: InterviewPhase
+  working_tree: Record<string, unknown> | null
+  tree_metadata: Record<string, unknown> | null
+}
+
+export interface AIChatSessionResponse {
+  session_id: string
+  status: 'active' | 'completed' | 'abandoned'
+  current_phase: InterviewPhase
+  flow_type: 'troubleshooting' | 'procedural'
+  conversation_history: ChatMessage[]
+  working_tree: Record<string, unknown> | null
+  tree_metadata: Record<string, unknown> | null
+  message_count: number
+  generated_tree: Record<string, unknown> | null
+}
+
+export interface AIChatGenerateResponse {
+  tree_structure: Record<string, unknown>
+  tree_metadata: Record<string, unknown>
+  status: string
+}
+
+export interface AIChatImportResponse {
+  tree_id: string
+  tree_type: string
+}

frontend/src/types/index.ts

@@ -52,3 +52,13 @@ export type {
   AIFixProposal,
   AIFixValidationError,
 } from './ai-fix'
+
+export type {
+  InterviewPhase,
+  ChatMessage,
+  AIChatStartResponse,
+  AIChatMessageResponse,
+  AIChatSessionResponse,
+  AIChatGenerateResponse,
+  AIChatImportResponse,
+} from './ai-chat'