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>

modules/repository/commits_test.go

@@ -4,14 +4,12 @@
 package repository
 
 import (
-	"strconv"
 	"testing"
 	"time"
 
 	repo_model "gitea.dev/models/repo"
 	"gitea.dev/models/unittest"
 	"gitea.dev/modules/git"
-	"gitea.dev/modules/setting"
 
 	"github.com/stretchr/testify/assert"
 )
@@ -99,38 +97,6 @@ func TestPushCommits_ToAPIPayloadCommits(t *testing.T) {
 	assert.Equal(t, []string{"readme.md"}, headCommit.Modified)
 }
 
-func TestPushCommits_AvatarLink(t *testing.T) {
-	assert.NoError(t, unittest.PrepareTestDatabase())
-
-	pushCommits := NewPushCommits()
-	pushCommits.Commits = []*PushCommit{
-		{
-			Sha1:           "abcdef1",
-			CommitterEmail: "user2@example.com",
-			CommitterName:  "User Two",
-			AuthorEmail:    "user4@example.com",
-			AuthorName:     "User Four",
-			Message:        "message1",
-		},
-		{
-			Sha1:           "abcdef2",
-			CommitterEmail: "user2@example.com",
-			CommitterName:  "User Two",
-			AuthorEmail:    "user2@example.com",
-			AuthorName:     "User Two",
-			Message:        "message2",
-		},
-	}
-
-	assert.Equal(t,
-		"/avatars/ab53a2911ddf9b4817ac01ddcd3d975f?size="+strconv.Itoa(28*setting.Avatar.RenderedSizeFactor),
-		pushCommits.AvatarLink(t.Context(), "user2@example.com"))
-
-	assert.Equal(t,
-		"/assets/img/avatar_default.png",
-		pushCommits.AvatarLink(t.Context(), "nonexistent@example.com"))
-}
-
 func TestCommitToPushCommit(t *testing.T) {
 	now := time.Now()
 	sig := &git.Signature{