Migrate from iOS 17 Observation to iOS 16-compatible Combine architecture.

Lower deployment target to iOS 16, replace @Observable with ObservableObject, add navigation and UI compatibility shims, and include login smoke UI tests.

Co-authored-by: Cursor <cursoragent@cursor.com>

suixinkan/App/Navigation/NavigationCompatibility.swift

@@ -0,0 +1,31 @@
+//
+//  NavigationCompatibility.swift
+//  suixinkan
+//
+//  Created by Codex on 2026/6/26.
+//
+
+import SwiftUI
+
+extension View {
+    /// iOS 16 compatible replacement for iOS 17's navigationDestination(item:).
+    func appNavigationDestination<Item, Destination: View>(
+        item: Binding<Item?>,
+        @ViewBuilder destination: @escaping (Item) -> Destination
+    ) -> some View {
+        navigationDestination(
+            isPresented: Binding(
+                get: { item.wrappedValue != nil },
+                set: { isPresented in
+                    if !isPresented {
+                        item.wrappedValue = nil
+                    }
+                }
+            )
+        ) {
+            if let value = item.wrappedValue {
+                destination(value)
+            }
+        }
+    }
+}