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>

web_src/css/repo.css

@@ -1386,8 +1386,7 @@ tbody.commit-list {
   vertical-align: baseline;
 }
 
-.message-wrapper,
-.author-wrapper {
+.message-wrapper {
   overflow: hidden;
   text-overflow: ellipsis;
   max-width: 100%;
@@ -1395,12 +1394,6 @@ tbody.commit-list {
   vertical-align: middle;
 }
 
-.author-wrapper {
-  max-width: 180px;
-  align-self: center;
-  white-space: nowrap;
-}
-
 .latest-commit .message-wrapper {
   max-width: calc(100% - 2.5rem);
 }
@@ -1415,9 +1408,6 @@ tbody.commit-list {
   tr.commit-list {
     width: 100%;
   }
-  .author-wrapper {
-    max-width: 80px;
-  }
 }
 
 @media (min-width: 768px) and (max-width: 991.98px) {