feat(apple): cover the window while the core boots

The core initializes asynchronously at launch, so for a moment the transfer lists
look empty and the app feels stalled. Show a full-window overlay (centered spinner
+ "Starting…") as the top layer of the root stack while coreState.isInitialized is
false; it fades out once the core is ready.

apple/VniDrop/App/RootView.swift

@@ -63,6 +63,14 @@ struct RootView: View {
 					onRefuse: approvals.refuse
 				)
 			}
+			.overlay {
+				// A small, unobtrusive indicator while the core finishes its async
+				// startup — otherwise the lists look empty and the app feels stalled.
+				if !sendModel.coreState.isInitialized {
+					CoreStartingOverlay()
+				}
+			}
+			.animation(.easeInOut(duration: 0.25), value: sendModel.coreState.isInitialized)
 			.vniDropTheme(isDark: isDark)
 			.preferredColorScheme(appModel.themeMode.preferredColorScheme)
 			.environment(\.vniColors, isDark ? .dark : .light)
@@ -183,6 +191,32 @@ struct RootView: View {
 	}
 }
 
+/// A full-window cover with a centered spinner shown while the core is starting.
+private struct CoreStartingOverlay: View {
+	var body: some View {
+		ZStack {
+			backgroundColor.ignoresSafeArea()
+			VStack(spacing: 16) {
+				ProgressView().controlSize(.large)
+				Text(String(localized: L10n.App.starting))
+					.font(.headline)
+					.foregroundStyle(.secondary)
+			}
+		}
+		.transition(.opacity)
+		.accessibilityElement(children: .combine)
+		.accessibilityLabel(Text(String(localized: L10n.App.starting)))
+	}
+
+	private var backgroundColor: Color {
+		#if os(iOS)
+		Color(uiColor: .systemBackground)
+		#else
+		Color(nsColor: .windowBackgroundColor)
+		#endif
+	}
+}
+
 #if os(iOS)
 import UIKit
 #else