fix(apple): show receiver-approval modal on macOS release builds

The approval modal never appeared for a macOS sender: the receiver request
reached the core and even fired its notification, but the modal stayed hidden.

Root cause was observation, not presentation. `RootView` derived `approvals`
and `messages` as `@ObservedObject` in `init` from a freshly built `AppGraph`.
`init` runs on every view re-creation and each run makes a throwaway graph, so
those observed objects were repointed to a dead `ApprovalCoordinator` that never
receives core events — while the persisted `@StateObject graph` (and the models
wired to it) kept the live one. Debug happened not to re-init the view, so it
stayed on the live instance; release re-inits it, exposing the bug.

Move the snackbar + approval modal into an `OverlayLayer` child view that takes
the coordinator/messages as `@ObservedObject` and is constructed in `body` from
the persisted `graph`, so the subscription is always against the live instances.

While here:
- Present the approval only after any open share/QR sheet has actually finished
  dismissing (macOS can't stack sheets), driven off the sheet's real
  `onDismiss` completion via a new `AdaptiveDrawer.onDismissed` hook and
  `SendModel.shareSheetsDismissed` — no wall-clock delay.
- Move the list-level share-sheet state (`shareTargetId`) into `SendModel` so the
  approval flow can dismiss every share surface centrally.
- Add a fallback: pending receiver rows in the Receivers panel now offer an
  Approve action (`SendModel.acceptReceiver`) alongside Refuse, for the case the
  modal didn't surface.

apple/VniDrop/UI/Components/AdaptiveDrawer.swift

@@ -7,11 +7,16 @@ struct AdaptiveDrawer<DrawerContent: View>: ViewModifier {
 	@Binding var isPresented: Bool
 	let windowClass: WindowClass
 	let onDismiss: () -> Void
+	/// Fired after the sheet's dismissal animation completes (as opposed to
+	/// `onDismiss`, which requests the close). Lets callers serialize a follow-up
+	/// sheet against this one's actual teardown instead of guessing a delay.
+	let onDismissed: (() -> Void)?
 	@ViewBuilder let drawerContent: () -> DrawerContent
 
 	func body(content: Content) -> some View {
 		content.sheet(
-			isPresented: Binding(get: { isPresented }, set: { if !$0 { onDismiss() } })
+			isPresented: Binding(get: { isPresented }, set: { if !$0 { onDismiss() } }),
+			onDismiss: onDismissed
 		) {
 			SheetChrome(onClose: onDismiss) { drawerContent() }
 				.modifier(PhoneDetents(enabled: windowClass == .phone))
@@ -56,11 +61,12 @@ extension View {
 		isPresented: Binding<Bool>,
 		windowClass: WindowClass,
 		onDismiss: @escaping () -> Void,
+		onDismissed: (() -> Void)? = nil,
 		@ViewBuilder content: @escaping () -> DrawerContent
 	) -> some View {
 		modifier(AdaptiveDrawer(
 			isPresented: isPresented, windowClass: windowClass,
-			onDismiss: onDismiss, drawerContent: content
+			onDismiss: onDismiss, onDismissed: onDismissed, drawerContent: content
 		))
 	}
 }