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/gituser/avatar_stack.go

@@ -0,0 +1,44 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package gituser
+
+import (
+	"context"
+
+	"gitea.dev/models/user"
+	"gitea.dev/modules/git"
+	"gitea.dev/modules/log"
+)
+
+// AvatarStackData is the view-model for the AvatarStack render helpers. Participants[0] is
+// the primary participant (commit author), painted on top; the rest follow.
+type AvatarStackData struct {
+	Participants      []*CommitParticipant
+	SearchByEmailLink string
+}
+
+func BuildAvatarStackData(ctx context.Context, allParticipants []*git.CommitIdentity, emailUserMap *user.EmailUserMap) *AvatarStackData {
+	if emailUserMap == nil {
+		emails := make([]string, len(allParticipants))
+		for i, sig := range allParticipants {
+			emails[i] = sig.Email
+		}
+		var err error
+		emailUserMap, err = user.GetUsersByEmails(ctx, emails)
+		if err != nil {
+			log.Error("GetUsersByEmails failed: %v", err)
+		}
+	}
+	ret := &AvatarStackData{
+		Participants: make([]*CommitParticipant, 0, len(allParticipants)),
+	}
+	for _, p := range allParticipants {
+		var giteaUser *user.User
+		if emailUserMap != nil {
+			giteaUser = emailUserMap.GetByEmail(p.Email)
+		}
+		ret.Participants = append(ret.Participants, &CommitParticipant{GiteaUser: giteaUser, GitIdentity: p})
+	}
+	return ret
+}