fix(repo): commit page fails to render unsigned commits with a different committer (#39381)

Since #39229 the commit page header dereferences
`.Verification.CommittingUser` when the committer is not the author.
`Verification` is `nil` for unsigned commits (see `repo.Diff`), so
opening such a commit — a rebased or cherry-picked one, for example —
logs a template error and the page comes out truncated:

```
Render failed: failed to render template: repo/commit_page, error: template error: builtin(bindata):repo/commit_page:138:22 : executing "repo/commit_page" at <.Verification.CommittingUser>: nil pointer evaluating interface {}.CommittingUser
```

This guards the access and adds an integration test that creates a
commit with distinct author and committer identities and checks the page
renders completely (the status stays 200 on a mid-render failure, so the
test looks at the body).

_The fix was worked out with help from an AI assistant; I reviewed and
tested it myself._

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

templates/repo/commit_page.tmpl

@@ -135,7 +135,7 @@
 				<span class="flex-text-inline tw-text-text-light">{{ctx.Locale.Tr "repo.diff.committed_at" $authors $committedAt}}</span>
 			{{else}}
 				{{$committerAvatar := ""}}{{$committerDisplayName := ""}}
-				{{if .Verification.CommittingUser}}
+				{{if and .Verification .Verification.CommittingUser}}
 					{{$committerAvatar = ctx.AvatarUtils.Avatar .Verification.CommittingUser 20}}
 					{{$committerDisplayName = HTMLFormat `%s%s` .Verification.CommittingUser.GetShortDisplayNameLinkHTML (ctx.RenderUtils.UserTypeLabel .Verification.CommittingUser)}}
 				{{else}}

templates/repo/settings/options.tmpl

@@ -907,7 +907,6 @@
 		</div>
 		{{end}}
 	</div>
-{{template "repo/settings/layout_footer" .}}
 
 {{if $.CanManagerDangerZone}}
 	{{if .Repository.IsMirror}}
@@ -1083,3 +1082,5 @@
 {{end}}
 
 {{template "repo/settings/push_mirror_sync_modal" .}}
+
+{{template "repo/settings/layout_footer" .}}