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="https://github.com/go-gitea/gitea/assets/115237/3a46cdbf-6af2-4bf9-b07f-021348badaac">

Mobile:
<img width="640" alt="Screenshot 2024-04-29 at 22 34 19"
src="https://github.com/go-gitea/gitea/assets/115237/8a2beea7-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/features/repo-code.js

@@ -113,6 +113,7 @@ function showLineButton() {
   btn.closest('.code-view').append(menu.cloneNode(true));
 
   createTippy(btn, {
+    theme: 'menu',
     trigger: 'click',
     hideOnClick: true,
     content: menu,