Accessibility and phone layout fixes

What changed

• Alerts and confirmations now open as in-page dialogs instead of browser pop-ups. Screen readers announce them, and closing one puts you back where you were.
• Wide tables such as match history, recruiter analytics, and candidate comparisons now scroll sideways on phones. Assessment workspaces fit the visible screen on phones, so Run and Submit stay in reach.
• No two problems in the library share a name anymore, and a few problems that were stored twice have been removed.