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/repo.css

@@ -1,3 +1,12 @@
+.repo-header {
+  margin: 1em 0 0.5em;
+}
+
+.repo-header .secondary-info {
+  margin-top: 0.5rem;
+  font-size: 12px;
+}
+
 .repository .data-table .line-num,
 .repository .diff-file-box .file-body.file-code .lines-num,
 .repository .diff-file-box .code-diff tbody tr .lines-type-marker {