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/issue_test.go

@@ -34,7 +34,7 @@ import (
 func getIssuesSelection(t testing.TB, htmlDoc *HTMLDoc) *goquery.Selection {
 	issueList := htmlDoc.doc.Find("#issue-list")
 	assert.Equal(t, 1, issueList.Length())
-	return issueList.Find(".flex-item").Find(".issue-title")
+	return issueList.Find(".item").Find(".issue-title")
 }
 
 func getIssue(t *testing.T, repoID int64, issueSelection *goquery.Selection) *issues_model.Issue {