fix(ui): ensure dimmer always covers whole page (#7471)

- Ensure that the dimmer always covers the whole page and that the modal is centered.
- Ensure that `body` hides overflow so you cannot scroll on the page after the modal is opened.
- The adjusted CSS 'behavior' originates from the original dimmer module.
- Regression of https://codeberg.org/forgejo/forgejo/pulls/7416.
- E2E test added.

Screenshots

Before:
https://codeberg.org/attachments/fb8c84b3-94ba-4597-b468-4bf344f356ed

After:
https://codeberg.org/attachments/a6583eb9-1ec7-4e40-960a-4986f6e17535

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/7471
Reviewed-by: Beowulf <beowulf@beocode.eu>
Reviewed-by: 0ko <0ko@noreply.codeberg.org>
Co-authored-by: Gusted <postmaster@gusted.xyz>
Co-committed-by: Gusted <postmaster@gusted.xyz>

web_src/css/modules/dimmer.css

@@ -1,3 +1,7 @@
+body:has(> .ui.active.dimmer) {
+  overflow: hidden;
+}
+
 .ui.active.dimmer {
   display: flex;
   opacity: 1;
@@ -10,8 +14,9 @@
   display: none;
   flex-direction: column;
   height: 100%;
-  position: absolute;
+  position: fixed;
   opacity: 0;
+  transform-origin: center center;
   justify-content: center;
   user-select: none;
   width: 100%;