feat: first native dialog for modal (#8859)

- The current implementation for modals is provided by fomantic UI.
- This patch introduces a new implementation that relies on the `<dialog>` element to provide modal, whereby the heavy lifting is done by the browser.
- This implementation is considerably simpler, accessible (although untested) and lightweight. It is capable of replacing fomantic UI's modal implementation + our dimmer implementation (~2k lines of code and CSS).[^1] As a first step the empty content modal is migrated.
- This brings in the CSS needed to display `<dialog>` and a helper function that hides some boilerplate code that's needed to show `<dialog>` as a modal.
- Add a E2E test that shows the modal's cancel and approve button works.

[^1]: The heavy work has already been done by me in a local branch, but reviewing that gigantic patch in one PR is not doable.

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/8859
Reviewed-by: 0ko <0ko@noreply.codeberg.org>
Co-authored-by: Gusted <postmaster@gusted.xyz>
Co-committed-by: Gusted <postmaster@gusted.xyz>

web_src/js/index.js

@@ -86,6 +86,7 @@ import {initDirAuto} from './modules/dirauto.js';
 import {initRepositorySearch} from './features/repo-search.js';
 import {initColorPickers} from './features/colorpicker.js';
 import {initRepoMilestoneEditor} from './features/repo-milestone.js';
+import {initModalClose} from './modules/modal.ts';
 
 // Init Gitea's Fomantic settings
 initGiteaFomantic();
@@ -189,6 +190,7 @@ onDomReady(() => {
   initRepoDiffView();
   initScopedAccessTokenCategories();
   initColorPickers();
+  initModalClose();
 
   // Deactivate CSS-only noJS usability supplements
   document.body.classList.remove('no-js');