Refactor "flex-list" to "flex-divided-list" (#37505)

Purpose:

1. Make the whole code base have unified "item" layout
2. Clarify our "list" styles: "flex-relaxed-list", "flex-divided-list"
3. Prepare to replace legacy "ui relaxed list"
* https://github.com/go-gitea/gitea/pull/37445#discussion_r3144458865
4. Prepare for refactoring the "pull merge box", it needs the
"flex-divided-list"
    * related to "Refactor pull request view (*)" like #37451
5. Fix legacy abuses of "flex-list", e.g.: repo home sidebar

web_src/css/actions.css

@@ -43,7 +43,7 @@
   align-items: center;
 }
 
-.run-list .flex-item-trailing {
+.run-list .item-trailing {
   flex-wrap: nowrap;
   width: 280px;
   flex: 0 0 280px;
@@ -55,7 +55,7 @@
 }
 
 @media (max-width: 767.98px) {
-  .run-list .flex-item-trailing {
+  .run-list .item-trailing {
     flex-direction: column;
     align-items: flex-end;
     width: auto;