Add session persistence across app restarts #2
Description
Problem
All sessions are lost when the app quits. Users have to recreate their tab layout every time.
Proposed Solution
Persist tab names, count, group structure, and layout mode to UserDefaults on quit. On launch, restore the session layout (new shell processes, but same names and grid arrangement).
No process state restore needed — just the workspace layout.
Impact
- Major UX improvement for daily use
- Minimal code: serialize/deserialize SessionStore state