Vendor relative-time-element as local web component (#36853)

Replace the `@github/relative-time-element` npm dependency with a
vendored, simplified implementation.

- Support 24h format rendering [PR
329](https://github.com/github/relative-time-element/pull/329)
- Enable `::selection` styling in Firefox [PR
341](https://github.com/github/relative-time-element/pull/341)
- Remove timezone from tooltips (It's always local timezone)
- Clean up previous `title` workaround in tippy
- Remove unused features
- Use native `Intl.DurationFormat` with fallback for older browsers,
remove dead polyfill
- Add MIT license header to vendored file
- Add unit tests
- Add dedicated devtest page for all component variants

---------

Signed-off-by: silverwind <me@silverwind.io>
Co-authored-by: Claude claude-opus-4-6 20250630 <noreply@anthropic.com>

web_src/css/base.css

@@ -238,7 +238,8 @@ progress::-moz-progress-bar {
   background: var(--color-hover);
 }
 
-::selection {
+::selection,
+relative-time::part(root)::selection {
   background: var(--color-primary-light-1);
   color: var(--color-white);
 }