Session persistence across page reloads

[datasciencemonkey]

Summary

Currently tabs and terminal sessions are lost on page refresh. Add session persistence so users can reconnect to running PTY processes after a reload.

Approach

This requires backend changes:

• PTY sessions survive disconnect (don't kill on close)
• Store tab layout in localStorage (tab IDs, labels, session IDs)
• On reload, reconnect to existing PTY sessions via stored session IDs
• Stale sessions cleaned up by existing heartbeat/timeout logic

Scope

This is a separate project from multi-tab support (#39). Requires backend PTY resumption support.