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/components/ai-chat/EmptyPreview.tsx

@@ -0,0 +1,13 @@
+import { TreeDeciduous } from 'lucide-react'
+
+export function EmptyPreview() {
+  return (
+    <div className="flex h-full flex-col items-center justify-center p-8 text-center">
+      <TreeDeciduous className="h-12 w-12 text-muted-foreground/30 mb-4" />
+      <h3 className="text-sm font-medium text-muted-foreground mb-1">Flow Preview</h3>
+      <p className="text-xs text-muted-foreground/70 max-w-48">
+        Your flow will appear here as you describe it to the AI
+      </p>
+    </div>
+  )
+}