style: collapse SAFE_NAVIGATION_URL_RE onto one line per prettier

prettier wanted the regex constant inline (it fits under the 80-char
print width). No behaviour change.

Note: the `pre-commit (previous)` check on this PR is expected to keep
failing — it lints the parent commit (5c0689dc95) which still has the
lint issues this branch later fixed. Squash-on-merge resolves it; not
worth force-pushing to flatten the history while iterating.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>