Rework and fix stopwatch (#30732)

Fixes https://github.com/go-gitea/gitea/issues/30721 and overhauls the
stopwatch. Time is now shown inside the "dot" icon and on both mobile
and desktop. All rendering is now done by `<relative-time>`, the
`pretty-ms` dependency is dropped.

Desktop:
<img width="557" alt="Screenshot 2024-04-29 at 22 33 27"
src="3a46cdbf-6af2-4bf9-b07f-021348badaac">

Mobile:
<img width="640" alt="Screenshot 2024-04-29 at 22 34 19"
src="8a2beea7-bd5d-473f-8fff-66f63fd50877">

Note for tippy:
Previously, tippy instances defaulted to "menu" theme, but that theme is
really only meant for `.ui.menu`, so it was not optimal for the
stopwatch popover.

This introduces a unopinionated `default` theme that has no padding and
should be suitable for all content. I reviewed all existing uses and
explicitely set the desired `theme` on all of them.

web_src/js/modules/tippy.js

@@ -37,8 +37,10 @@ export function createTippy(target, opts = {}) {
       return onShow?.(instance);
     },
     arrow: arrow || (theme === 'bare' ? false : arrowSvg),
-    role: role || 'menu', // HTML role attribute
-    theme: theme || role || 'menu', // CSS theme, either "tooltip", "menu", "box-with-header" or "bare"
+    // HTML role attribute, ideally the default role would be "popover" but it does not exist
+    role: role || 'menu',
+    // CSS theme, either "default", "tooltip", "menu", "box-with-header" or "bare"
+    theme: theme || role || 'default',
     plugins: [followCursor],
     ...other,
   });