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>

models/user/user.go

@@ -1148,14 +1148,7 @@ func GetUsersBySource(ctx context.Context, s *auth.Source) ([]*User, error) {
 	return users, err
 }
 
-// UserCommit represents a commit with validation of user.
-type UserCommit struct { //revive:disable-line:exported
-	User *User
-	*git.Commit
-}
-
-// ValidateCommitWithEmail check if author's e-mail of commit is corresponding to a user.
-func ValidateCommitWithEmail(ctx context.Context, c *git.Commit) *User {
+func GetUserByGitAuthor(ctx context.Context, c *git.Commit) *User {
 	if c.Author == nil {
 		return nil
 	}
@@ -1166,33 +1159,6 @@ func ValidateCommitWithEmail(ctx context.Context, c *git.Commit) *User {
 	return u
 }
 
-// ValidateCommitsWithEmails checks if authors' e-mails of commits are corresponding to users.
-func ValidateCommitsWithEmails(ctx context.Context, oldCommits []*git.Commit) ([]*UserCommit, error) {
-	var (
-		newCommits = make([]*UserCommit, 0, len(oldCommits))
-		emailSet   = make(container.Set[string])
-	)
-	for _, c := range oldCommits {
-		if c.Author != nil {
-			emailSet.Add(c.Author.Email)
-		}
-	}
-
-	emailUserMap, err := GetUsersByEmails(ctx, emailSet.Values())
-	if err != nil {
-		return nil, err
-	}
-
-	for _, c := range oldCommits {
-		user := emailUserMap.GetByEmail(c.Author.Email) // FIXME: why ValidateCommitsWithEmails uses "Author", but ParseCommitsWithSignature uses "Committer"?
-		newCommits = append(newCommits, &UserCommit{
-			User:   user,
-			Commit: c,
-		})
-	}
-	return newCommits, nil
-}
-
 type EmailUserMap struct {
 	m map[string]*User
 }
@@ -1203,7 +1169,7 @@ func (eum *EmailUserMap) GetByEmail(email string) *User {
 
 func GetUsersByEmails(ctx context.Context, emails []string) (*EmailUserMap, error) {
 	if len(emails) == 0 {
-		return nil, nil //nolint:nilnil // return nil when there are no emails to look up
+		return &EmailUserMap{}, nil
 	}
 
 	needCheckEmails := make(container.Set[string])