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>

suixinkanUITests/LoginSmokeUITests.swift

@@ -0,0 +1,28 @@
+//
+//  LoginSmokeUITests.swift
+//  suixinkanUITests
+//
+//  Created by Codex on 2026/6/26.
+//
+
+import XCTest
+
+final class LoginSmokeUITests: XCTestCase {
+    override func setUp() {
+        super.setUp()
+        continueAfterFailure = false
+    }
+
+    func testFreshLaunchShowsLoginScreen() {
+        let app = XCUIApplication()
+        app.launchArguments.append("-suixinkan-ui-tests-reset-state")
+        app.launch()
+
+        XCTAssertTrue(app.staticTexts["login.title"].waitForExistence(timeout: 5))
+        XCTAssertTrue(app.textFields["login.username"].exists)
+        XCTAssertTrue(app.secureTextFields["login.password"].exists || app.textFields["login.password"].exists)
+        XCTAssertTrue(app.buttons["login.submit"].exists)
+        XCTAssertTrue(app.buttons["login.userAgreement"].exists)
+        XCTAssertTrue(app.buttons["login.privacyPolicy"].exists)
+    }
+}