feat: confirm before exiting CCManager

[kbwo]

Summary

• Exiting CCManager from the menu or the multi-project list terminated every active session immediately, with no way to back out of an accidental keypress.
• Add a confirmation dialog before exit, defaulting focus to "Cancel", that also reports how many sessions would be terminated.
• Confirming still exits and terminates sessions as before; canceling returns to the screen the exit was triggered from. Sessions terminated this way remain restorable on the next launch, unchanged from existing behavior.

Problem

Both exit entry points — the single-project menu's "Exit" action (q/x or selecting the row) and the multi-project list's "Exit" row — called globalSessionOrchestrator.destroyAllSessions() and exit() directly. A stray keypress or a wrong selection immediately killed every running session with no confirmation step, unlike other destructive actions in the app (e.g. deleting a worktree), which already prompt first.

Verification

Prerequisites:

• None beyond the existing dev setup (bun install).

Steps:

• bun run lint
• bun run typecheck
• bun run test (full suite, run twice to check for flakiness) — all pass
• New tests in src/components/App.test.tsx cover: canceling returns to the menu without destroying sessions, confirming destroys sessions and exits, and the same confirm flow from the multi-project list