fix: enable GFM extension for markdown rendering in proxy mode

When `filter_readme_scripts` is enabled, markdown files served through
the proxy are converted to HTML using goldmark. However, the default
goldmark converter does not include GFM (GitHub Flavored Markdown)
extensions, causing tables, strikethrough, and other GFM syntax to be
rendered as plain text instead of proper HTML elements.

This adds `extension.GFM` to the goldmark converter so that GFM tables,
strikethrough, autolinks, and task lists are correctly converted to HTML.

Author: icebear0828

server/handles/down.go

@@ -18,6 +18,7 @@ import (
 	"github.com/microcosm-cc/bluemonday"
 	log "github.com/sirupsen/logrus"
 	"github.com/yuin/goldmark"
+	"github.com/yuin/goldmark/extension"
 )
 
 func Down(c *gin.Context) {
@@ -151,7 +152,8 @@ func localProxy(c *gin.Context, link *model.Link, file model.Obj, proxyRange boo
 			}
 
 			var html bytes.Buffer
-			if err = goldmark.Convert(buf.Bytes(), &html); err != nil {
+			md := goldmark.New(goldmark.WithExtensions(extension.GFM))
+			if err = md.Convert(buf.Bytes(), &html); err != nil {
 				err = fmt.Errorf("markdown conversion failed: %w", err)
 			} else {
 				buf.Reset()