Update settings sticky header/footer

What does this MR do and why?

Updates to the settings sticky header and footer

• Changes shadows to 1px gray border, border color matches section dividers
• Slightly reduce sticky header padding
• Remove sticky behavior when only one section exists

Screenshots or screen recordings

Before	After

How to set up and validate locally

• Go to a settings page with multiple sections (eg http://localhost:3000/-/profile/preferences); header should be sticky
• Go to a settings page with a single section (eg http://localhost:3000/-/profile/password/edit); header should be static

UX Paper Cuts 16.3 → Settings UI, part 2 (#417912 - closed)