Replace blocking confirm() dialog with custom non-blocking modal to prevent
browser warning issues when users have "don't show warnings" enabled.
Changes:
- Add showConfirmModal() method with Promise-based async modal
- Update closeSession() to use non-blocking modal
- Update deleteSession() to use non-blocking modal
- Add complete CSS styling for confirmation modal
- Support keyboard (Escape key) and click-outside to close
- Responsive design for mobile devices
- Dark theme matching existing UI
Fixes issue where close button completely stopped working after browser
blocked confirm() dialog and user selected "don't show warnings".
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>