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>

options/locale/locale_en-US.json

@@ -2205,10 +2205,10 @@
   "repo.settings.trust_model.collaborator.desc": "Valid signatures by collaborators of this repository will be marked \"trusted\", whether they match the committer or not. Otherwise, valid signatures will be marked \"untrusted\" if the signature matches the committer and \"unmatched\" if not.",
   "repo.settings.trust_model.committer": "Committer",
   "repo.settings.trust_model.committer.long": "Committer: Trust signatures that match committers. This matches GitHub's behavior and will force commits signed by Gitea to have Gitea as the committer.",
-  "repo.settings.trust_model.committer.desc": "Valid signatures will only be marked \"trusted\" if they match the committer, otherwise they will be marked \"unmatched\". This forces Gitea to be the committer on signed commits, with the actual committer marked as Co-authored-by: and Co-committed-by: trailer in the commit. The default Gitea key must match a user in the database.",
+  "repo.settings.trust_model.committer.desc": "Valid signatures will only be marked \"trusted\" if they match the committer, otherwise they will be marked \"unmatched\". This forces Gitea to be the committer on signed commits, with the actual committer marked as a Co-authored-by: trailer in the commit. The default Gitea key must match a user in the database.",
   "repo.settings.trust_model.collaboratorcommitter": "Collaborator+Committer",
   "repo.settings.trust_model.collaboratorcommitter.long": "Collaborator+Committer: Trust signatures by collaborators which match the committer",
-  "repo.settings.trust_model.collaboratorcommitter.desc": "Valid signatures by collaborators of this repository will be marked \"trusted\" if they match the committer. Otherwise, valid signatures will be marked \"untrusted\" if the signature matches the committer and \"unmatched\" otherwise. This will force Gitea to be marked as the committer on signed commits, with the actual committer marked as Co-Authored-By: and Co-Committed-By: trailer in the commit. The default Gitea key must match a user in the database.",
+  "repo.settings.trust_model.collaboratorcommitter.desc": "Valid signatures by collaborators of this repository will be marked \"trusted\" if they match the committer. Otherwise, valid signatures will be marked \"untrusted\" if the signature matches the committer and \"unmatched\" otherwise. This will force Gitea to be marked as the committer on signed commits, with the actual committer marked as a Co-Authored-By: trailer in the commit. The default Gitea key must match a user in the database.",
   "repo.settings.wiki_delete": "Delete Wiki Data",
   "repo.settings.wiki_delete_desc": "Deleting repository wiki data is permanent and cannot be undone.",
   "repo.settings.wiki_delete_notices_1": "- This will permanently delete and disable the repository wiki for %s.",
@@ -2599,6 +2599,9 @@
   "repo.diff.review.reject": "Request changes",
   "repo.diff.review.self_approve": "Pull request authors can't approve their own pull request",
   "repo.diff.committed_by": "committed by",
+  "repo.diff.coauthored_by": "co-authored by",
+  "repo.commits.avatar_stack_and": "and",
+  "repo.commits.avatar_stack_people": "%d people",
   "repo.diff.protected": "Protected",
   "repo.diff.image.side_by_side": "Side by Side",
   "repo.diff.image.swipe": "Swipe",