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>

backend/app/api/endpoints/ai_chat.py

@@ -387,12 +387,8 @@ async def import_tree(
             detail="Session must be completed with a generated tree before importing",
         )
 
-    if session.generated_tree_id:
-        return AIChatImportResponse(
-            tree_id=session.generated_tree_id,
-            tree_type=session.flow_type,
-        )
-
+    # Always create a new Tree record (no duplicate check — user may
+    # want multiple copies or re-import after edits)
     metadata = session.tree_metadata or {}
     tree = Tree(
         name=data.name or metadata.get("name", "AI-Generated Flow"),