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/js/features/repo-settings-branches.test.ts

@@ -13,13 +13,13 @@ vi.mock('../modules/sortable.ts', () => ({
 
 const branchesHTML = `
   <div id="protected-branches-list" data-update-priority-url="some/repo/branches/priority">
-    <div class="flex-item tw-items-center item" data-id="1">
+    <div class="item" data-id="1">
       <div class="drag-handle"></div>
     </div>
-    <div class="flex-item tw-items-center item" data-id="2">
+    <div class="item" data-id="2">
       <div class="drag-handle"></div>
     </div>
-    <div class="flex-item tw-items-center item" data-id="3">
+    <div class="item" data-id="3">
       <div class="drag-handle"></div>
     </div>
   </div>