feat(tui): add persistent session terminals

[jlongster]

Summary

• render a single fixed session frame with the session on the left and the selected persistent terminal on the right
• manage terminal membership and selection through session-scoped SessionTerminals state; no recursive pane trees or arbitrary splits
• preserve the session component while switching terminals
• restore terminal snapshots and replay, synchronize control/input/resizing, and reconnect through ticketed WebSockets
• provide terminal selection, new-terminal creation, keyboard pane focus, live terminal/process titles, and focus-aware prompt styling
• reconcile session terminals when other clients add or remove them
• preserve terminal text selections without copying empty click selections

Verification

• TUI and CLI typechecks
• 25 focused session-tab and selection tests

Includes the former #44973 polish changes.