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/components/ai-chat/ChatToolbar.tsx

@@ -1,4 +1,4 @@
-import { Sparkles, Download, RotateCcw, ArrowRight } from 'lucide-react'
+import { Sparkles, Save, RotateCcw, Loader2 } from 'lucide-react'
 import { PhaseIndicator } from './PhaseIndicator'
 import { cn } from '@/lib/utils'
 import type { InterviewPhase } from '@/types'
@@ -8,8 +8,9 @@ interface ChatToolbarProps {
   status: 'idle' | 'active' | 'completed' | 'abandoned'
   isGenerating: boolean
   hasGeneratedTree: boolean
+  isSaving: boolean
   onGenerate: () => void
-  onImport: () => void
+  onSave: () => void
   onReset: () => void
 }
 
@@ -18,8 +19,9 @@ export function ChatToolbar({
   status,
   isGenerating,
   hasGeneratedTree,
+  isSaving,
   onGenerate,
-  onImport,
+  onSave,
   onReset,
 }: ChatToolbarProps) {
   return (
@@ -44,22 +46,42 @@ export function ChatToolbar({
               'disabled:opacity-50 disabled:cursor-not-allowed'
             )}
           >
-            <Download className="h-3.5 w-3.5" />
-            {isGenerating ? 'Generating...' : 'Generate Flow'}
+            {isGenerating ? (
+              <>
+                <Loader2 className="h-3.5 w-3.5 animate-spin" />
+                Generating...
+              </>
+            ) : (
+              <>
+                <Sparkles className="h-3.5 w-3.5" />
+                Generate Flow
+              </>
+            )}
           </button>
         )}
 
         {hasGeneratedTree && (
           <button
-            onClick={onImport}
+            onClick={onSave}
+            disabled={isSaving}
             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'
+              'hover:opacity-90 transition-opacity',
+              'disabled:opacity-50 disabled:cursor-not-allowed'
             )}
           >
-            <ArrowRight className="h-3.5 w-3.5" />
-            Import to Editor
+            {isSaving ? (
+              <>
+                <Loader2 className="h-3.5 w-3.5 animate-spin" />
+                Saving...
+              </>
+            ) : (
+              <>
+                <Save className="h-3.5 w-3.5" />
+                Save to Flow Library
+              </>
+            )}
           </button>
         )}