refactor: replace Fomantic modal with native dialog (#39846)

Replace Fomantic's hard-forked jQuery modal with native `<dialog>`
elements, advancing https://github.com/go-gitea/gitea/issues/29849. The
UI is visually unchanged. Dialogs open with `show()` over an `inert`
page, so tooltips, popups and toasts mounted on `<body>` stay usable
above them, unlike with `showModal()`
(https://github.com/whatwg/html/issues/9936).

- Remove the Fomantic modal and dimmer, add `showModal` and `hideModal`
- Declare modals as `<dialog class="modal-dialog">`, shown inside a
`.modal-backdrop`, instead of `ui modal` with its unused size classes
- Remove two modals that could never be opened
- Block the page while a modal is open, close the topmost one on Escape
and restore focus to its opener
- Allow stacked modals, use a confirm modal instead of `window.confirm`
for deleting content history
- Hide popups and toasts when a modal opens or closes
- Keep the release notes modal open when generating notes fails

Fixes: https://github.com/go-gitea/gitea/issues/39643
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

web_src/css/modules/transition.css

@@ -1,23 +1,12 @@
-/* styles for Fomantic transition (toggle): dimmer, modal, dropdown menu */
+/* styles for Fomantic transition (toggle): dropdown menu */
 
 /* this is the only place using "hidden" and "visible" classes, it can safely work with tailwind without prefix in the future */
-.ui.modal.transition.hidden,
 .ui.dropdown > .menu.transition.hidden {
   display: none;
   visibility: hidden;
 }
 
-.ui.modal.transition.visible,
 .ui.dropdown > .menu.transition.visible {
   display: block;
   visibility: visible;
 }
-
-/* dimmer uses "active" but not "hidden/visible" classes, no special classes for it here */
-
-/* only modal and dimmer need animation, dropdown menu doesn't */
-.ui.modal.transition {
-  animation-name: fadein;
-  animation-duration: 100ms;
-  animation-timing-function: ease-in-out;
-}