feat(widgets): add native Android and iOS home screen players

Publish artwork, metadata, and transport state through a shared Dart bridge. Add configurable Android RemoteViews and iOS WidgetKit players with controls and links into the full player.

Serialize widget commands and session restoration, and share one headless-capable iOS Flutter engine with the foreground scene. Include native integration tests, Dart bridge tests, and widget setup documentation.

ios/PlayerWidget/PlayerControlIntent.swift

@@ -0,0 +1,19 @@
+import AppIntents
+
+@available(iOS 17.0, *)
+struct PlayerControlIntent: AudioPlaybackIntent {
+    static var title: LocalizedStringResource = "Control music playback"
+    static var openAppWhenRun = false
+
+    @Parameter(title: "Action") var action: String
+
+    init() { action = "toggle" }
+    init(_ action: String) { self.action = action }
+
+    func perform() async throws -> some IntentResult {
+        #if !PLAYER_WIDGET_EXTENSION
+        _ = await PlayerWidgetBridge.shared.perform(action)
+        #endif
+        return .result()
+    }
+}