[codex] keep Drift listening in the menubar

[vsamarth]

What changed

• Added a desktop tray/menubar lifecycle layer so Drift can hide instead of quitting on close.
• Kept receiver discovery enabled while the app is running, so the listener stays active even when the window is not on the home route.
• Made incoming offers restore/focus the app and open the existing receive-transfer UI.
• Added a focused test covering offers that arrive while the app is on Settings.

Why

The goal is to let Drift keep listening for file transfers from the menubar without building a larger background-mode/settings workflow yet.

Validation

• flutter test test/app_router_test.dart
• flutter test test/features/receive/feature_test.dart test/features/transfers/service_test.dart
• flutter analyze
• flutter build macos --debug

Notes

• The PR intentionally stays minimal: it keeps listening and surfaces incoming offers in the UI, but does not add a dedicated background-listening toggle yet.