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/dimmer.css

@@ -1,27 +1,24 @@
-/* These are the remnants of the fomantic dimmer module */
-
-.ui.dimmer {
+.modal-backdrop {
   position: fixed;
   display: none;
   inset: 0;
   background: var(--color-overlay-backdrop);
-  opacity: 0;
-  z-index: 1000;
+  z-index: var(--z-index-modal);
   overflow-y: auto;
   justify-content: center;
   padding: 8px 0;
   animation-name: fadein;
   animation-duration: .2s;
+  overscroll-behavior: contain;
   user-select: none;
 }
 
-.ui.active.dimmer {
+.modal-backdrop:has(> [open]) {
   display: flex;
-  opacity: 1;
 }
 
-.ui.dimmer > .ui.modal {
+.modal-backdrop > .modal-dialog {
   position: static;
-  margin-top: auto !important;
-  margin-bottom: auto !important;
+  margin-top: auto;
+  margin-bottom: auto;
 }