Mac 原生系统通知 + 接收目录可配（Phase 4b·1）

- MacNotifier：窗口非前台（!NSApp.isActive）时对「收到消息 / 收到文件」发原生系统通知（UNUserNotificationCenter，授权已由引擎启动的 PushRegistry.requestAuthorizationAndRegister 一并请求）。对齐桌面 app.ShowNotification——前台事件由应用内 NoticeOverlay / 未读红点负责，仅 app 非活跃才发系统通知，避免与应用内提示重复。引擎 handleNotify 的 "message"（新到对端消息，非当前会话）+ "transferDone"（incoming 且 completed）两处加 macOS 挂钩
- 接收目录可配：DownloadManager.downloadDir 在 macOS 读 UserDefaults macDownloadDir（设置页 NSOpenPanel 选目录），未配 / 目录已失效则回退系统「下载」目录（比「文稿」更符桌面直觉）；iOS 仍固定沙盒 Documents。设置加「接收」区（显示当前保存目录 + 选择目录…）。非沙盒构建用普通路径即可；沙盒分发（Phase 6）须改安全作用域书签
- 验证：just mac-build（CommilitiaDropMac.app）+ iOS 模拟器（CommilitiaDrop.app）均 BUILD SUCCEEDED

ios/CDrop/MacApp/MacRootView.swift

@@ -1,4 +1,5 @@
 #if os(macOS)
+import AppKit
 import SwiftUI
 
 // 桌面根界面：NavigationSplitView 侧栏（传输 / 文件 / 消息 / 设备 / 设置）+ 详情 NavigationStack，
@@ -210,6 +211,7 @@ struct MacSettingsView: View
     @Environment(EngineController.self) private var engine
     @Environment(AuthManager.self) private var auth
     @AppStorage("macClipboardAutoSync") private var autoSync = true
+    @AppStorage("macDownloadDir") private var downloadDirPath = ""
     @State private var nameDraft = DeviceNameStore.value
 
     private var signalingLabel: String
@@ -245,6 +247,11 @@ struct MacSettingsView: View
                 Button("上传本机剪贴板") { engine.uploadClipboard() }
                 Button("拉取云剪贴板到本机") { engine.pullClipboard() }
             }
+            Section("接收")
+            {
+                LabeledContent("保存到", value: downloadDirDisplay)
+                Button("选择目录…") { pickDownloadDir() }
+            }
             Section("引擎")
             {
                 LabeledContent("状态", value: engine.status)
@@ -280,5 +287,23 @@ struct MacSettingsView: View
         guard !name.isEmpty else { return }
         engine.renameSelf(to: name)
     }
+
+    private var downloadDirDisplay: String
+    {
+        downloadDirPath.isEmpty ? "下载（默认）" : (downloadDirPath as NSString).abbreviatingWithTildeInPath
+    }
+
+    private func pickDownloadDir()
+    {
+        let panel = NSOpenPanel()
+        panel.canChooseFiles = false
+        panel.canChooseDirectories = true
+        panel.allowsMultipleSelection = false
+        panel.prompt = "选择"
+        if panel.runModal() == .OK, let url = panel.url
+        {
+            downloadDirPath = url.path
+        }
+    }
 }
 #endif