fix: replace Import button with "Save to Flow Library" and remove duplicate check

- After generation, toolbar shows "Save to Flow Library" button
  (replaces "Import to Editor")
- Button shows "Saving..." spinner state during API call
- Generate button shows animated spinner during generation
- Backend /import endpoint always creates a new Tree record
  (removed generated_tree_id idempotency check)
- Navigates to tree editor after successful save

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/pages/AIChatBuilderPage.tsx

@@ -1,4 +1,4 @@
-import { useCallback, useEffect } from 'react'
+import { useCallback, useEffect, useState } from 'react'
 import { useNavigate, useSearchParams } from 'react-router-dom'
 import { useAIChatStore } from '@/store/aiChatStore'
 import { ChatPanel } from '@/components/ai-chat/ChatPanel'
@@ -62,11 +62,15 @@ export function AIChatBuilderPage() {
     [sendMessage]
   )
 
+  const [isSaving, setIsSaving] = useState(false)
+
   const handleGenerate = useCallback(() => {
     generateTree()
   }, [generateTree])
 
-  const handleImport = useCallback(async () => {
+  const handleSave = useCallback(async () => {
+    if (isSaving) return
+    setIsSaving(true)
     try {
       const treeId = await importToEditor({
         name: treeMetadata?.name,
@@ -75,11 +79,13 @@ export function AIChatBuilderPage() {
       })
       const path = getTreeEditorPath(treeId, flowType)
       navigate(path)
-      toast.success('Flow imported to editor')
+      toast.success('Flow saved to library')
     } catch {
-      toast.error('Failed to import flow')
+      toast.error('Failed to save flow')
+    } finally {
+      setIsSaving(false)
     }
-  }, [importToEditor, treeMetadata, flowType, navigate])
+  }, [isSaving, importToEditor, treeMetadata, flowType, navigate])
 
   const handleReset = useCallback(async () => {
     await abandonSession()
@@ -116,8 +122,9 @@ export function AIChatBuilderPage() {
         status={status}
         isGenerating={isGenerating}
         hasGeneratedTree={!!generatedTree}
+        isSaving={isSaving}
         onGenerate={handleGenerate}
-        onImport={handleImport}
+        onSave={handleSave}
         onReset={handleReset}
       />