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/templates/util_render_test.go

@@ -7,15 +7,19 @@ import (
 	"context"
 	"html/template"
 	"os"
+	"strconv"
 	"strings"
 	"testing"
 
+	"gitea.dev/models/gituser"
 	"gitea.dev/models/issues"
 	"gitea.dev/models/repo"
 	user_model "gitea.dev/models/user"
+	"gitea.dev/modules/git"
 	"gitea.dev/modules/markup"
 	"gitea.dev/modules/reqctx"
 	"gitea.dev/modules/setting"
+	"gitea.dev/modules/setting/config"
 	"gitea.dev/modules/test"
 	"gitea.dev/modules/translation"
 
@@ -298,3 +302,52 @@ func TestUserMention(t *testing.T) {
 	rendered := newTestRenderUtils(t).MarkdownToHtml("@no-such-user @mention-user @mention-user")
 	assert.Equal(t, `<p>@no-such-user <a href="/mention-user" rel="nofollow">@mention-user</a> <a href="/mention-user" rel="nofollow">@mention-user</a></p>`, strings.TrimSpace(string(rendered)))
 }
+
+func TestAvatarStack(t *testing.T) {
+	defer test.MockVariableValue(&config.SkipDatabaseConfig, true)()
+
+	ut := newTestRenderUtils(t)
+	mkCo := func(name, email string) *git.CommitIdentity {
+		return &git.CommitIdentity{Name: name, Email: email}
+	}
+	authorSig := mkCo("Alice", "alice@example.com")
+	mkData := func(co ...*git.CommitIdentity) *gituser.AvatarStackData {
+		all := append([]*git.CommitIdentity{authorSig}, co...)
+		return gituser.BuildAvatarStackData(t.Context(), all, &user_model.EmailUserMap{})
+	}
+
+	t.Run("lone author renders bare name, no label", func(t *testing.T) {
+		got := string(ut.AvatarStackWithNames(mkData()))
+		assert.Contains(t, got, `<span class="avatar-stack-names">`)
+		assert.Contains(t, got, "Alice")
+		assert.NotContains(t, got, "avatar_stack_and")
+		assert.NotContains(t, got, "avatar_stack_people")
+	})
+
+	t.Run("two participants use and label", func(t *testing.T) {
+		got := string(ut.AvatarStackWithNames(mkData(mkCo("Bob", "bob@example.com"))))
+		assert.Contains(t, got, "repo.commits.avatar_stack_and")
+		assert.Contains(t, got, "Bob")
+		assert.NotContains(t, got, "avatar_stack_people")
+		assert.Contains(t, got, `<span class="avatar-stack">`)
+	})
+
+	t.Run("three participants switch to N people label with tippy popup", func(t *testing.T) {
+		got := string(ut.AvatarStackWithNames(mkData(mkCo("Bob", "bob@example.com"), mkCo("Carol", "carol@example.com"))))
+		assert.Contains(t, got, "repo.commits.avatar_stack_people:3")
+		assert.NotContains(t, got, "repo.commits.avatar_stack_and")
+		assert.Contains(t, got, `data-global-init="initAvatarStackPopup"`)
+		assert.Contains(t, got, `<div class="tippy-target">`)
+		assert.Contains(t, got, `class="avatar-stack-popup"`)
+	})
+
+	t.Run("overflow chip renders beyond 10 participants", func(t *testing.T) {
+		cos := make([]*git.CommitIdentity, 0, renderAvatarStackMaxVisible+1)
+		for i := range renderAvatarStackMaxVisible + 1 {
+			cos = append(cos, mkCo("X", strconv.Itoa(i)+"@example.com"))
+		}
+		got := ut.AvatarStack(gituser.BuildAvatarStackData(t.Context(), cos, &user_model.EmailUserMap{}))
+		assert.Contains(t, got, `class="avatar-stack-overflow-chip`)
+		assert.Contains(t, got, "+1")
+	})
+}