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/e2e/issue-project.test.ts

@@ -335,7 +335,7 @@ test('filter issues with no project using project=-1', async ({page}) => {
     await expect(page.locator('#issue-list')).not.toContainText('Issue with project assigned');
 
     // Verify the last item in the list is NOT the issue with a project
-    const issueItems = page.locator('#issue-list .flex-item');
+    const issueItems = page.locator('#issue-list .item');
     const lastIssueItem = issueItems.last();
     await expect(lastIssueItem).not.toContainText('Issue with project assigned');
   } finally {