Multi-meeting session model + switching

[YJack0000]

Goal

Let Parley manage multiple joined meetings at once and switch between them. Because each meeting produces its own live transcript, the user can hop between meetings and instantly pick up where each one is.

Today the store is single-meeting (one segments/evaluations/todos set). This introduces a session model: each joined meeting = a session with its own transcript, evaluations, todos, summary, and browser instance.

Tasks

• Session model: Session { id, title, status, segments, evaluations, todos, summary, browserHandle }; refactor the store to hold N sessions + an activeSessionId (keep the existing single-session UX as one session).
• Run N concurrent browser/caption sessions (one per joined meeting); route caption segments to the right session by id.
• Reuse the evaluation engine + next-move per session (independent timers).
• Switch active session → the UI shows that meeting's transcript / summary / evals.
• Resource awareness: cap concurrent sessions; surface CPU/RAM cost of each browser instance.

Notes

• This is the architectural backbone for the "manage two meetings and switch between them" idea.
• The MCP session.json and usage logging should become session-aware too (follow-up).

Part of the "Multi-meeting co-pilot & catch-up mode" epic.