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/State/ScenicSpotContext.swift

@@ -6,7 +6,7 @@
 //
 
 import Foundation
-import Observation
+import Combine
 
 /// 景点加载阶段实体，表示当前景区景点列表的加载状态。
 enum ScenicSpotLoadState: Equatable {
@@ -17,12 +17,11 @@ enum ScenicSpotLoadState: Equatable {
 }
 
 @MainActor
-@Observable
 /// 景点上下文状态中心，保存当前景区下的景点或打卡点列表。
-final class ScenicSpotContext {
-    private(set) var scenicId: Int?
-    private(set) var spots: [ScenicSpotItem] = []
-    private(set) var loadState: ScenicSpotLoadState = .idle
+final class ScenicSpotContext: ObservableObject {
+    @Published private(set) var scenicId: Int?
+    @Published private(set) var spots: [ScenicSpotItem] = []
+    @Published private(set) var loadState: ScenicSpotLoadState = .idle
 
     /// 按景区 ID 重新加载景点列表，失败只影响景点模块本身。
     func reload(scenicId: Int?, api: AccountContextServing) async {