- Add window.matchMedia('(max-width: 767px)') phone check
- Desktop branch: position fixed, top/left 50%, translate(-50%, -50%), maxWidth 480px, borderRadius 12px
- Phone branch: unchanged bottom-sheet (bottom 0, left 0, right 0, borderRadius 12px 12px 0 0)
- role=dialog, aria-modal, aria-label unchanged — only position CSS branched
- SettingsSheet, ChangePasswordSheet, LinkOidcSheet centering in prior commit (Task 1)