Add native macOS confirmation before quitting or closing bb #3706

The work

Add optional native macOS confirmation dialogs before quitting bb or closing a window.

• Quit: Cover ⌘Q, bb → Quit, and Dock → Quit with Cancel / Quit.
• Close window: Cover the red close button with Cancel / Close Window.
• Provide separate settings for each confirmation.
• Default to Cancel and support Escape.
• Show confirmation before any runtime shutdown or cleanup. Cancelling must leave agents, terminals, and connections running normally.
• Avoid duplicate prompts and handle update restarts and macOS shutdown appropriately.

Why now

It’s easy to accidentally close or quit bb while agents are working. A confirmation would help prevent unintended interruptions.

Plugins can’t currently solve this for me. The plugin API exposes no hook to cancel application quit, and a window-level close blocker would run after bb has already started shutting down its runtime. This needs support in the desktop app so cancelling actually keeps everything running.

Acceptance criteria

Acceptance criteria

• Separate settings control confirmation before quitting and closing a window, and persist across restarts.
• When enabled, ⌘Q, bb → Quit, and Dock → Quit show a native macOS Cancel / Quit dialog.
• When enabled, the red close button shows a native Cancel / Close Window dialog.
• Cancel is the default; Escape cancels.
• Confirmation appears before runtime shutdown or cleanup begins.
• Cancelling leaves windows, agents, terminals, and connections functioning normally.
• Confirming proceeds with the existing quit or window-close behavior.
• Repeated requests do not create duplicate dialogs.
• Update restarts and macOS shutdown/logout do not get stuck behind confirmation prompts.
• When disabled, existing behavior is preserved.