Refactor pull request view (7) (#37524)

Almost done

`pull_merge_box.tmpl` only has about 80 lines now, and (almost) all
variable accesses are strictly typed.

---------

Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: Claude (Opus 4.7) <noreply@anthropic.com>
Co-authored-by: Nicolas <bircni@icloud.com>

options/locale/locale_en-US.json

@@ -1958,7 +1958,6 @@
   "repo.signing.wont_sign.headsigned": "The merge will not be signed as the head commit is not signed.",
   "repo.signing.wont_sign.commitssigned": "The merge will not be signed as all the associated commits are not signed.",
   "repo.signing.wont_sign.approved": "The merge will not be signed as the PR is not approved.",
-  "repo.signing.wont_sign.not_signed_in": "You are not signed in.",
   "repo.ext_wiki": "Access to External Wiki",
   "repo.ext_wiki.desc": "Link to an external wiki.",
   "repo.wiki": "Wiki",