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/repo_fork_test.go

@@ -82,7 +82,7 @@ func TestRepoForkToOrg(t *testing.T) {
 
 func TestForkListLimitedAndPrivateRepos(t *testing.T) {
 	defer tests.PrepareTestEnv(t)()
-	forkItemSelector := ".fork-list .flex-item"
+	forkItemSelector := ".fork-list .item"
 
 	user1Sess := loginUser(t, "user1")
 	user1 := unittest.AssertExistsAndLoadBean(t, &user_model.User{Name: "user1"})