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

tests/integration/privateactivity_test.go

@@ -57,7 +57,7 @@ func testPrivateActivityHelperEnablePrivateActivity(t *testing.T) {
 }
 
 func testPrivateActivityHelperHasVisibleActivitiesInHTMLDoc(htmlDoc *HTMLDoc) bool {
-	return htmlDoc.doc.Find("#activity-feed").Find(".flex-item").Length() > 0
+	return htmlDoc.doc.Find("#activity-feed").Find(".item").Length() > 0
 }
 
 func testPrivateActivityHelperHasVisibleActivitiesFromSession(t *testing.T, session *TestSession) bool {