accessibility: Modal dialogs don't trap focus — keyboard user can tab outside

[sshdopey]

Problem

Focus escapes modal on Tab, user gets stuck in background content.

[Fhatu12]

Hi, I'd like to work on this issue. I’ll keep the fix focused on the modal dialog path that still lacks focus trapping/restoration (EmailAuthModal), add regression coverage for Tab/Shift+Tab staying inside the dialog, and avoid broad modal rewrites or live wallet/Stellar flows. Please assign me before I proceed.