chore: core 多模块骨架

IosClientKit/Sources/CoreUI/EmptyStateView.swift

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+import SwiftUI
+
+/// 空状态占位视图。
+public struct EmptyStateView: View {
+    private let title: String
+    private let systemImage: String
+
+    public init(title: String, systemImage: String = "tray") {
+        self.title = title
+        self.systemImage = systemImage
+    }
+
+    public var body: some View {
+        ContentUnavailableView(title, systemImage: systemImage)
+    }
+}

IosClientKit/Sources/CoreUI/ErrorView.swift

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+import SwiftUI
+
+/// 错误占位视图（带重试）。
+public struct ErrorView: View {
+    private let message: String
+    private let retry: () -> Void
+
+    public init(message: String, retry: @escaping () -> Void) {
+        self.message = message
+        self.retry = retry
+    }
+
+    public var body: some View {
+        ContentUnavailableView {
+            Label("出错了", systemImage: "exclamationmark.triangle")
+        } description: {
+            Text(message)
+        } actions: {
+            Button("重试", action: retry)
+        }
+    }
+}

IosClientKit/Sources/CoreUI/LoadingView.swift

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+import SwiftUI
+
+/// 加载中占位视图。
+public struct LoadingView: View {
+    public init() {}
+
+    public var body: some View {
+        ProgressView("加载中…")
+            .frame(maxWidth: .infinity, maxHeight: .infinity)
+    }
+}

IosClientKit/Sources/CoreUI/PlaceholderView.swift

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+import SwiftUI
+
+/// 尚未实现的 Tab 占位视图。
+public struct PlaceholderView: View {
+    private let title: String
+    private let systemImage: String
+
+    public init(_ title: String, systemImage: String) {
+        self.title = title
+        self.systemImage = systemImage
+    }
+
+    public var body: some View {
+        ContentUnavailableView(title, systemImage: systemImage)
+    }
+}