Bug/Feature: View Multiple Sessions w/show

Prior versions of playwright-cli showed multiple sessions at once, allowing a user to monitor all of those sessions at once. The new update makes show much nicer and more functional for individual sessions, but removed the ability to monitor multiple sessions at once.

Suggested UX: allow selecting multiple sessions in the left sidebar. If there are multiple sessions selected, display those sessions in the right side in a flex layout that scales the views to the pane size. Add a "Select All" button to next to the sessions gear. Clicking on a session changes the selection to that single session.

Optionally, add an "auto select" session button that selects the sessions that have activity in the last five minutes. Selecting an individual session deselects the auto select. When auto select is active, only show active sessions.

Start with the playwright-cli show interface and trace how sessions are selected in the left sidebar and rendered in the right pane. Implement the described multi-selection, Select All behavior, single-session replacement, and responsive flex layout; treat auto-select as optional. Done means multiple sessions can be monitored together without breaking individual-session selection.