- Add tabindex="-1" to #scrollbox in submit_form/show so focus lands
there after skip link activation (not just scroll position)
- Add skip link to plain layout (used by submissions/show)
- Add id="main-content" tabindex="-1" to submissions/show outer div
as the skip link target
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>