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>

options/locale/locale_en-US.json

@@ -1519,6 +1519,7 @@
   "repo.issues.commented_at": "commented <a href=\"#%s\">%s</a>",
   "repo.issues.delete_comment_confirm": "Are you sure you want to delete this comment?",
   "repo.issues.context.copy_link": "Copy Link",
+  "repo.issues.context.copy_source": "Copy Source",
   "repo.issues.context.quote_reply": "Quote Reply",
   "repo.issues.context.reference_issue": "Reference in New Issue",
   "repo.issues.context.edit": "Edit",