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/ChatToolbar.tsx

@@ -0,0 +1,76 @@
+import { Sparkles, Download, RotateCcw, ArrowRight } from 'lucide-react'
+import { PhaseIndicator } from './PhaseIndicator'
+import { cn } from '@/lib/utils'
+import type { InterviewPhase } from '@/types'
+
+interface ChatToolbarProps {
+  currentPhase: InterviewPhase
+  status: 'idle' | 'active' | 'completed' | 'abandoned'
+  isGenerating: boolean
+  hasGeneratedTree: boolean
+  onGenerate: () => void
+  onImport: () => void
+  onReset: () => void
+}
+
+export function ChatToolbar({
+  currentPhase,
+  status,
+  isGenerating,
+  hasGeneratedTree,
+  onGenerate,
+  onImport,
+  onReset,
+}: ChatToolbarProps) {
+  return (
+    <div className="flex items-center justify-between border-b border-border bg-card px-4 py-2">
+      <div className="flex items-center gap-3">
+        <div className="flex items-center gap-2 text-sm font-medium text-foreground">
+          <Sparkles className="h-4 w-4 text-primary" />
+          Build with AI
+        </div>
+        <PhaseIndicator currentPhase={currentPhase} />
+      </div>
+
+      <div className="flex items-center gap-2">
+        {status === 'active' && !hasGeneratedTree && (
+          <button
+            onClick={onGenerate}
+            disabled={isGenerating}
+            className={cn(
+              'flex items-center gap-1.5 rounded-lg px-3 py-1.5 text-sm font-medium',
+              'bg-gradient-brand text-white shadow-lg shadow-primary/20',
+              'hover:opacity-90 transition-opacity',
+              'disabled:opacity-50 disabled:cursor-not-allowed'
+            )}
+          >
+            <Download className="h-3.5 w-3.5" />
+            {isGenerating ? 'Generating...' : 'Generate Flow'}
+          </button>
+        )}
+
+        {hasGeneratedTree && (
+          <button
+            onClick={onImport}
+            className={cn(
+              'flex items-center gap-1.5 rounded-lg px-3 py-1.5 text-sm font-medium',
+              'bg-gradient-brand text-white shadow-lg shadow-primary/20',
+              'hover:opacity-90 transition-opacity'
+            )}
+          >
+            <ArrowRight className="h-3.5 w-3.5" />
+            Import to Editor
+          </button>
+        )}
+
+        <button
+          onClick={onReset}
+          className="flex items-center gap-1.5 rounded-lg border border-border px-3 py-1.5 text-sm text-muted-foreground hover:bg-accent hover:text-foreground transition-colors"
+        >
+          <RotateCcw className="h-3.5 w-3.5" />
+          Start Over
+        </button>
+      </div>
+    </div>
+  )
+}