fix: update push commit data paths for Gitea 1.27.1 compatibility

Gitea 1.27.1 fixed the mail template for push events (#38467) —
commit data must now be accessed via .UserCommit.GitCommit instead
of directly on the commit object.

Changed across all 10 themes:
- {{.ID}}               → {{.UserCommit.GitCommit.ID}}
- {{.ID.String}}        → {{.UserCommit.GitCommit.ID.String}}
- {{.MessageTitle}}     → {{.UserCommit.GitCommit.MessageTitle}}

Verification:
- 3 mail-related files changed in Gitea v1.27.0→v1.27.1
- All 110 templates (10 themes × 11 types) render successfully
- No breaking changes — data structure unchanged, only access path

Co-Authored-By: Claude <noreply@anthropic.com>

themes/terminal/mail/repo/issue/default.tmpl

@@ -140,7 +140,7 @@
               <ul>
                 {{range .Comment.Commits}}
                   <li>
-                    <a href="{{$.Comment.Issue.PullRequest.BaseRepo.HTMLURL}}/commit/{{.ID}}">{{ShortSha .ID.String}}</a> - {{.MessageTitle}}
+                    <a href="{{$.Comment.Issue.PullRequest.BaseRepo.HTMLURL}}/commit/{{.UserCommit.GitCommit.ID}}">{{ShortSha .UserCommit.GitCommit.ID.String}}</a> - {{.UserCommit.GitCommit.MessageTitle}}
                   </li>
                 {{end}}
               </ul>