feat(app): secrets kept out of screenshots on macOS and Windows too

The reading of the full reading's fixes noted that macOS and Windows can
keep a window out of screen captures, which SecretScreen didn't ask.

PlatformScreenGuard now asks the app's window on Android, macOS and
Windows (asksOn), and both apps' runners answer app.halkyon/screen_guard,
counting what shows: macOS's MainFlutterWindow sets sharingType = .none,
Windows' FlutterWindow WDA_EXCLUDEFROMCAPTURE (Windows 10 2004 on;
before, WDA_MONITOR, black in captures). Neither runner was built here
(Linux): to be tried on each system.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

apps/todo/halkyon_todo_flutter/macos/Runner/MainFlutterWindow.swift

@@ -2,6 +2,13 @@ import Cocoa
 import FlutterMacOS
 
 class MainFlutterWindow: NSWindow {
+  /// The channel halkyon_app's SecretScreen asks to keep the window out of
+  /// screenshots and screen recordings while something secret shows (the
+  /// recovery phrase, recovery codes, an authenticator's key), and how many
+  /// show: counted, as several may at once.
+  private var screenGuard: FlutterMethodChannel?
+  private var secretsShown = 0
+
   override func awakeFromNib() {
     let flutterViewController = FlutterViewController()
     let windowFrame = self.frame
@@ -9,7 +16,34 @@ class MainFlutterWindow: NSWindow {
     self.setFrame(windowFrame, display: true)
 
     RegisterGeneratedPlugins(registry: flutterViewController)
+    guardScreen(messenger: flutterViewController.engine.binaryMessenger)
 
     super.awakeFromNib()
   }
+
+  private func guardScreen(messenger: FlutterBinaryMessenger) {
+    let channel = FlutterMethodChannel(
+      name: "app.halkyon/screen_guard", binaryMessenger: messenger)
+    channel.setMethodCallHandler { [weak self] call, result in
+      guard let self = self else {
+        result(nil)
+        return
+      }
+      switch call.method {
+      case "hold":
+        self.secretsShown += 1
+        self.sharingType = .none
+        result(nil)
+      case "release":
+        self.secretsShown = max(0, self.secretsShown - 1)
+        if self.secretsShown == 0 {
+          self.sharingType = .readOnly
+        }
+        result(nil)
+      default:
+        result(FlutterMethodNotImplemented)
+      }
+    }
+    screenGuard = channel
+  }
 }