feat: Migrate to React Flow and add Fixed + Dynamic category mode

Frontend:
- Migrate MindmapDAG from D3.js to React Flow (@xyflow/react)
- Add custom node components (QueryNode, CategoryHeaderNode, AttributeNode)
- Add useDAGLayout hook for column-based layout
- Add "AI" badge for LLM-suggested categories
- Update CategorySelector with Fixed + Dynamic mode option
- Improve dark/light theme support

Backend:
- Add FIXED_PLUS_DYNAMIC category mode
- Filter duplicate category names in LLM suggestions
- Update prompts to exclude fixed categories when suggesting new ones
- Improve LLM service with better error handling and logging
- Auto-remove /no_think prefix for non-Qwen models
- Add smart JSON format detection for model compatibility
- Improve JSON extraction with multiple parsing strategies

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

frontend/package.json

@@ -12,6 +12,7 @@
   "dependencies": {
     "@ant-design/icons": "^6.1.0",
     "@types/d3": "^7.4.3",
+    "@xyflow/react": "^12.9.3",
     "antd": "^6.0.0",
     "d3": "^7.9.0",
     "react": "^19.2.0",