Add “Copy Source” to markup comment menu (#36726)

Any user with **read access** to a comment can now copy its raw markdown
source via the `···` context menu — no edit permission required.

Closes #36722.

---------

Signed-off-by: silverwind <me@silverwind.io>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

templates/repo/issue/view_content/context_menu.tmpl

@@ -10,6 +10,7 @@
 			{{$referenceUrl = printf "%s/files#%s" ctx.RootData.Issue.Link .item.HashTag}}
 		{{end}}
 		<div class="item context js-aria-clickable" data-clipboard-text-type="url" data-clipboard-text="{{$referenceUrl}}">{{ctx.Locale.Tr "repo.issues.context.copy_link"}}</div>
+		<div class="item context js-aria-clickable" data-clipboard-target="#{{.item.HashTag}}-raw">{{ctx.Locale.Tr "repo.issues.context.copy_source"}}</div>
 		{{if ctx.RootData.IsSigned}}
 			{{$needDivider := false}}
 			{{if not ctx.RootData.Repository.IsArchived}}