Allow fast-forward-only merge when signed commits are required (#37335)

Fast-forward-only creates no Gitea commit, so skip the "can Gitea sign"
precheck for it. Pre-check head-commit verification for styles that
preserve user commits on the target (merge, fast-forward-only) so a PR
with unsigned commits surfaces a localized error instead of a 500 at the
pre-receive hook. The dropdown still shows every configured style; the
avatar and signing warning toggle per selection via
data-pull-merge-style.

Fixes #12272 

**Note**: Admin force-merge does not bypass the new head-commits check.
This matches the existing `isSignedIfRequired` behavior.

Signed-off-by: Nikita Vakula <programmistov.programmist@gmail.com>
Signed-off-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: Claude (Opus 4.7) <noreply@anthropic.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

options/locale/locale_en-US.json

@@ -1850,6 +1850,7 @@
   "repo.pulls.merge_manually": "Manually merged",
   "repo.pulls.merge_commit_id": "The merge commit ID",
   "repo.pulls.require_signed_wont_sign": "The branch requires signed commits but this merge will not be signed",
+  "repo.pulls.require_signed_head_commits_unverified": "The branch requires signed commits but one or more commits on this pull request are not verified",
   "repo.pulls.invalid_merge_option": "You cannot use this merge option for this pull request.",
   "repo.pulls.merge_conflict": "Merge Failed: There was a conflict while merging. Hint: Try a different strategy.",
   "repo.pulls.merge_conflict_summary": "Error Message",