Use CSS `inset` shorthand (#30939)

Use [inset](https://developer.mozilla.org/en-US/docs/Web/CSS/inset)
shorthand instead of longhands. There may be more cases but these ones I
was able to definitely identify.

web_src/css/helpers.css

@@ -20,10 +20,7 @@ Gitea's private styles use `g-` prefix.
 
 .g-table-auto-ellipsis td.auto-ellipsis span {
   position: absolute;
-  left: 0;
-  right: 0;
-  top: 0;
-  bottom: 0;
+  inset: 0;
   padding: inherit;
   white-space: nowrap;
   overflow: hidden;

web_src/css/markup/content.css

@@ -204,10 +204,7 @@
 
 .markup input[type="checkbox"]::after {
   position: absolute;
-  left: 0;
-  top: 0;
-  bottom: 0;
-  right: 0;
+  inset: 0;
   pointer-events: none;
   background: var(--color-text);
   mask-size: cover;

web_src/css/modules/dimmer.css

@@ -3,10 +3,7 @@
 .ui.dimmer {
   position: fixed;
   display: none;
-  top: 0;
-  left: 0;
-  right: 0;
-  bottom: 0;
+  inset: 0;
   background: var(--color-overlay-backdrop);
   opacity: 0;
   z-index: 1000;