refactor(biji): 重构笔记保存逻辑并修复相关BUG

- 后端：将笔记创建和更新逻辑合并为一个接口，根据ID是否存在自动判断操作类型
- 前端：修复了新建笔记时自动保存导致数据重复创建的问题
- 优化了与后端的数据同步，确保本地状态与服务器保持一致

biji-houdaun/src/main/java/com/test/bijihoudaun/controller/MarkdownController.java

@@ -41,25 +41,6 @@ public class MarkdownController {
         return R.fail();
     }
 
-    @Operation(summary = "创建markdown文件")
-    @Parameters({
-            @Parameter(name = "groupingId", description = "分组id",required = true),
-            @Parameter(name = "title", description = "标题",required = true),
-            @Parameter(name = "fileName", description = "文件名",required = true),
-            @Parameter(name = "content", description = "内容",required = true)
-    })
-    @PostMapping
-    public R<MarkdownFile> createMarkdown(
-            @RequestParam Long groupingId,
-            @RequestParam String title,
-            @RequestParam String fileName,
-            @RequestBody String content) {
-
-        MarkdownFile file = markdownFileService.createMarkdownFile(
-                groupingId, title, fileName, content);
-
-        return R.success(file);
-    }
 
     @Operation(summary = "更新Markdown文件")
     @PostMapping("/updateMarkdown")