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>

tests/e2e/utils.ts

@@ -91,6 +91,20 @@ export async function apiCloseIssue(requestContext: APIRequestContext, owner: st
   }), 'apiCloseIssue');
 }
 
+export async function apiEditIssue(requestContext: APIRequestContext, owner: string, repo: string, issueIndex: number, data: Record<string, unknown>) {
+  await apiRetry(() => requestContext.patch(`${baseUrl()}/api/v1/repos/${owner}/${repo}/issues/${issueIndex}`, {
+    headers: apiHeaders(),
+    data,
+  }), 'apiEditIssue');
+}
+
+export async function apiCreateBranch(requestContext: APIRequestContext, owner: string, repo: string, name: string) {
+  await apiRetry(() => requestContext.post(`${baseUrl()}/api/v1/repos/${owner}/${repo}/branches`, {
+    headers: apiHeaders(),
+    data: {new_branch_name: name},
+  }), 'apiCreateBranch');
+}
+
 /** Create a PR via API. Returns the PR index for subsequent operations. */
 export async function apiCreatePR(requestContext: APIRequestContext, owner: string, repo: string, head: string, base: string, title: string, {headers}: {headers?: Record<string, string>} = {}): Promise<number> {
   const response = await apiRetry(() => requestContext.post(`${baseUrl()}/api/v1/repos/${owner}/${repo}/pulls`, {