- Remove auto-selection of the most recent session on initial load,
so the app opens to a blank new-chat page instead of the last session.
- Preserve active session state when navigating to/from settings:
keep ThreadShell mounted (hidden via CSS) so scroll position, message
cache, and streaming state are not lost.
- Update onBackToChat to return to blank page when no session was active
instead of falling back to the most recent session.
- Update related test expectations to match the new navigation behavior.
彭星杰andXubin Ren
00597fccd6