Replace onboarding flow with silent v1 import and iCloud alert

Delete the multi-page onboarding sheet. On first launch the app now
silently imports any v1 instances from UserDefaults (splitting embedded
basic-auth credentials out of the URL and into the Keychain) and then,
if the device is signed in to iCloud, shows a single alert offering to
enable sync. Accepting shows a blocking progress overlay until the
initial upload completes.

Yattee/Views/ICloudSyncProgressView.swift

@@ -0,0 +1,49 @@
+//
+//  ICloudSyncProgressView.swift
+//  Yattee
+//
+//  Blocking progress overlay shown during the first-launch iCloud sync.
+//
+
+import SwiftUI
+
+struct ICloudSyncProgressView: View {
+    @Environment(\.appEnvironment) private var appEnvironment
+
+    private var cloudKitSync: CloudKitSyncEngine? { appEnvironment?.cloudKitSync }
+
+    var body: some View {
+        VStack(spacing: 24) {
+            ProgressView()
+                .controlSize(.large)
+
+            Text(String(localized: "onboarding.cloud.syncing.title"))
+                .font(.title2)
+                .fontWeight(.semibold)
+
+            Text(progressText)
+                .font(.subheadline)
+                .foregroundStyle(.secondary)
+                .multilineTextAlignment(.center)
+                .frame(maxWidth: 400)
+        }
+        .padding(40)
+        .frame(maxWidth: .infinity, maxHeight: .infinity)
+        #if os(macOS)
+        .frame(minWidth: 420, minHeight: 260)
+        #endif
+        .interactiveDismissDisabled()
+    }
+
+    private var progressText: String {
+        if let upload = cloudKitSync?.uploadProgress {
+            return upload.displayText
+        }
+        return String(localized: "onboarding.cloud.syncing.preparing")
+    }
+}
+
+#Preview {
+    ICloudSyncProgressView()
+        .appEnvironment(.preview)
+}