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
1. correct the modal usage on "admin email list" page (then
`web_src/js/features/admin/emails.ts` is removed)
2. use `addDelegatedEventListener` instead of `jQuery().on`
3. more jQuery related changes and remove jQuery from
`web_src/js/features/common-button.ts`
4. improve `confirmModal` to make it support header, and remove
incorrect double-escaping
5. fix more typescript related types
6. fine tune devtest pages and add more tests
