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/auth_ldap_test.go

@@ -395,7 +395,7 @@ func testLDAPUserSyncSSHKeys(t *testing.T) {
 		resp := session.MakeRequest(t, req, http.StatusOK)
 		htmlDoc := NewHTMLParser(t, resp.Body)
 
-		divs := htmlDoc.doc.Find("#keys-ssh .flex-item .flex-item-body:not(:last-child)")
+		divs := htmlDoc.doc.Find("#keys-ssh .item .item-body:not(:last-child)")
 		syncedKeys := make([]string, divs.Length())
 		for i := 0; i < divs.Length(); i++ {
 			syncedKeys[i] = strings.TrimSpace(divs.Eq(i).Text())