Fix flex container width (go-gitea#28603) (go-gitea#28605)

Backport go-gitea#28603 by wxiaoguang

Fix go-gitea#28489

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

web_src/css/modules/flexcontainer.css

@@ -11,6 +11,7 @@
 
 .flex-container-main {
   flex: 1;
+  min-width: 0; /* make the "text truncate" work, otherwise the flex axis is not limited and the text just overflows */
 }
 
 @media (max-width: 767.98px) {