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;