refactor(editor): migrate to Milkdown with LaTeX support and clean up legacy code

- Removed old contenteditable-based MarkdownEditor component
- Integrated Milkdown Crepe with LaTeX (KaTeX) rendering support
- Simplified inline suggestion plugin using ProseMirror decorations
- Removed debug logging and unused components (HelloWorld, plan files)
- Increased debounce from 150ms to 500ms for better performance
- Fixed SSE JSON serialization in backend main.py

backend/main.py

@@ -123,9 +123,8 @@ async def create_completion(request: CompletionRequest):
         # 返回完整内容
         async def generate():
             if content:
-                print(f"[LLM] Yielding full content: {repr(content)}")
                 yield f"data: {json.dumps({'content': content})}\n\n"
-            yield f"data: {{'done': true}}\n\n"
+            yield f"data: {json.dumps({'done': true})}\n\n"
         
         return StreamingResponse(generate(), media_type="text/event-stream")