Extend issue context popup beyond markdown content (#36908)

Extend the issue context popup beyond markdown. Any link rendered with
the `ref-issue` class now gets the popup, which covers commit titles and
issue titles everywhere they appear (repo home, commits list, blame,
branches, graph, PR commits, issue/PR pages, compare, …). For surfaces
that synthesize links without markdown autolinking (dashboard activity
feed, pulse page, commit merged-PR line), opt in by adding
`data-ref-issue-container` on a parent (or `ref-issue` on the link).

- Use `html_url` from the backend payload instead of synthesizing links
client-side
- Fetch outside the component, stateless, with a per-URL cache
- Small hover delay so passing over a link doesn't fire a request
- Drop the loading state (shifted layout)
- Make both links in the tooltip work; prevent nested tooltips
- Fix feed title `<a>` width so the tooltip only shows on link hover

Co-authored-by: Claude (Opus 4.6) <noreply@anthropic.com>

templates/repo/activity.tmpl

@@ -5,7 +5,7 @@
 		<div class="flex-container-nav">
 			{{template "repo/navbar" .}}
 		</div>
-		<div class="flex-container-main">
+		<div class="flex-container-main" data-ref-issue-container>
 			{{if .PageIsPulse}}{{template "repo/pulse" .}}{{end}}
 			{{if .PageIsContributors}}{{template "repo/contributors" .}}{{end}}
 			{{if .PageIsCodeFrequency}}{{template "repo/code_frequency" .}}{{end}}