Keep terminal mounted with an exit banner when a session dies

[HaydnG]

Summary

• When a pre-launch command (or claude itself) fails fast — wrong flag, missing binary, immediate stderr — the renderer's `onProcessExited` handler was calling `destroySession` immediately, which disposes the xterm instance and removes the DOM element. Anything the failing command wrote to stderr (`claude: command not found`, `devbox: unknown flag`, shell errors) was torn down with the terminal, leaving the user with no feedback at all — a terminal opens for a moment and vanishes.
• Now, on `process-exited`, the renderer writes a styled `── session exited (code N) — re-click this session in the sidebar to relaunch, or click another to dismiss ──` banner into the terminal and defers the destroy. The terminal stays mounted so the user can scroll back through whatever was printed before the exit.
• Re-clicking the session destroys + relaunches via the existing closed-entry branch in `openSession` (`public/app.js:779`).
• Plain terminal sessions stay ephemeral (destroyed and removed from the sidebar immediately on exit).

Why this matters

This was triggered for me by an org-specific misconfiguration (using `devbox ai --cli` as a pre-launch command, which is actually an installer not a launcher — it exited non-zero and the terminal vanished), but it's a generic UX cliff for any failed launch: misspelled `preLaunchCmd`, missing `aws-vault` profile, etc.

Test plan

• Set a pre-launch command to something that fails fast (e.g. `exit 1 &&`). Launch a session. Banner appears with code 1; terminal stays visible.
• Set a pre-launch command to something that prints to stderr then exits (e.g. `echo "boom" >&2; exit 2;`). Banner shows code 2; "boom" is still readable above the banner.
• Normal session exit (claude exits cleanly): banner shows code 0 with dim styling; terminal stays visible.
• Re-click the failed session in the sidebar: terminal destroyed + reopened via the closed-entry branch.
• Plain terminal: exit still removes it from the sidebar (no regression).