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/js/modules/fomantic.ts

@@ -1,7 +1,5 @@
 import {initAriaDropdownPatch} from './fomantic/dropdown.ts';
-import {initAriaModalPatch} from './fomantic/modal.ts';
 import {initFomanticTransition} from './fomantic/transition.ts';
-import {initFomanticDimmer} from './fomantic/dimmer.ts';
 import {svg} from '../svg.ts';
 
 export const fomanticMobileScreen = window.matchMedia('only screen and (max-width: 767.98px)');
@@ -20,9 +18,7 @@ export function initGiteaFomantic() {
   };
 
   initFomanticTransition();
-  initFomanticDimmer();
 
   // Use the patches to improve accessibility, these patches are designed to be as independent as possible, make it easy to modify or remove in the future.
   initAriaDropdownPatch();
-  initAriaModalPatch();
 }