Adds background notifications for the "the thing you were waiting for is
done" moments, alongside the existing incoming-approval-request one:
- a receive finished downloading (receive -> done)
- a receive failed / was interrupted (receive -> failed)
- a share you own failed (send -> failed)
- a receiver finished downloading your share (receiver status completed)
A new TransferNotificationCoordinator observes core state + signals and
publishes these; the decision of which moments notify is a pure function
(plannedTransferNotifications / plannedReceiverNotifications), unit-tested
independently. The first state snapshot only primes existing history as seen
so launch doesn't spam.
Notification permission is now the single source of truth. The in-app
notifications toggle and its decoupled UserDefaults preference are gone;
the Settings section shows an "Allow notifications" button that requests the
OS permission (or deep-links to Settings once decided), and notifications
gate purely on `permission == .granted`.
macOS delivery fixes:
- add a UNUserNotificationCenterDelegate so banners present even while the
app is active (the app window is usually open on macOS)
- present-when-active on macOS, suppress-when-foregrounded on iOS
- reserve the notification id before awaiting publish: the CombineLatest
fired several times and re-added the same identifier, which macOS
coalesces into a silent update with no banner
- LocalNotificationService seeds its permission at init so gating can't
race a not-yet-refreshed .notDetermined
Eight localized title/body strings added (apple-only); the shared
notifications_description copy is generalized from "receive requests" to
"transfer activity".
Add a native SwiftUI VniDrop app (Send/Receive/Settings) talking to the
Rust core via generated UniFFI Swift bindings, plus the uniffi-bindgen
helper crate. iOS uses a TabView, macOS a NavigationSplitView sidebar.
