Fix native macOS termination handling

[johanntan]

Summary

• update the locked wxDragon revision to one containing its macOS termination lifecycle hooks
• approve native macOS termination requests through AppEvents::on_should_terminate
• preserve the existing behavior where an ordinary red-button window close hides Paperback

Root cause

wxWidgets checks whether a macOS application may terminate by sending a vetoable close event to its top window. Paperback treated every vetoable macOS close as an ordinary window close, vetoed it, and hid the frame. This also cancelled shutdown/restart and Quit requests sent from the Dock or Application Switcher.

Current wxDragon main exposes OSXOnShouldTerminate through on_should_terminate, allowing Paperback to distinguish application termination without relying on Cocoa internals.

Validation

• cargo +nightly fmt --check
• cargo build -p paperback
• cargo check -p paperback --locked
• cargo test -p paperback (40 passed)
• native macOS Apple Event termination smoke test; callback ran and the process exited with status 0