feat: Add avatar stacks (#37594)

Parse `Co-authored-by:` trailers from commit messages and surface
contributors as an avatar stack across the commit page, commits list, PR
commits tab, latest-commit row, blame, graph, and dashboard feed.

- Up to 10 visible 20px avatars, GitHub-style overlap (6px first stride,
4px between subsequent), `+N` chip for the rest.
- Label: 1 → name; 2 → `<a> and <b>`; 3+ → `<N> people` opens a Tippy
popup with all participants.
- Names and avatars link to the repo's commits-by-author search; fall
back to profile or `mailto:`.
- Trailer parsing uses `net/mail.ParseAddress`, scans only the trailing
paragraph, filters out the commit's own author/committer.
- Drops the non-standard `Co-committed-by:` emission on squash merge and
web edits.

Devtest: `/devtest/coauthor-avatars`.

Fixes #25521

----
<img width="353" height="277" alt="image"
src="https://github.com/user-attachments/assets/72092ceb-97ca-4b09-9557-0b72d3c5458e"
/>

<img width="533" height="328"
src="https://github.com/user-attachments/assets/11d0c8f8-8b3f-4f2e-9993-879f1c06bcc5"
/>

---------

Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>
Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: Giteabot <teabot@gitea.io>

templates/user/dashboard/feeds.tmpl

@@ -89,10 +89,10 @@
 					{{$repo := .Repo}}
 					<div class="tw-flex tw-flex-col tw-gap-1">
 						{{range $pushCommit := $push.Commits}}
-							{{$commitLink := printf "%s/commit/%s" $repoLink .Sha1}}
+							{{$commitLink := printf "%s/commit/%s" $repoLink $pushCommit.Sha1}}
 							<div class="flex-text-block">
-								<img loading="lazy" alt class="ui avatar" src="{{$push.AvatarLink ctx .AuthorEmail}}" title="{{.AuthorName}}" width="16" height="16">
-								<a class="ui sha label" href="{{$commitLink}}">{{ShortSha .Sha1}}</a>
+								{{ctx.RenderUtils.AvatarStackPushCommit $pushCommit}}
+								<a class="ui sha label" href="{{$commitLink}}">{{ShortSha $pushCommit.Sha1}}</a>
 								<span class="tw-inline-block tw-truncate">
 									{{ctx.RenderUtils.RenderCommitMessage $pushCommit.Message $repo}}
 								</span>