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/PlayerWidgetState.swift

@@ -0,0 +1,45 @@
+import Foundation
+
+struct PlayerWidgetState: Codable {
+    var id = ""
+    var title = ""
+    var artist = ""
+    var album = ""
+    var playing = false
+    var canPlay = false
+    var canPrevious = false
+    var canNext = false
+    var background: UInt32 = 0xff292433
+    var artworkKey = ""
+    var artworkName = ""
+
+    static let kind = "SpotiFLACPlayer"
+    static var container: URL? {
+        let group = Bundle.main.object(forInfoDictionaryKey: "PlayerWidgetAppGroup") as? String
+            ?? "group.com.zarz.spotiflac"
+        return FileManager.default.containerURL(forSecurityApplicationGroupIdentifier: group)
+    }
+
+    static func read() -> PlayerWidgetState {
+        guard let url = container?.appendingPathComponent("player.json"),
+              let data = try? Data(contentsOf: url),
+              let state = try? JSONDecoder().decode(Self.self, from: data) else { return Self() }
+        return state
+    }
+
+    var artworkURL: URL? {
+        guard !artworkName.isEmpty, !artworkName.contains("/") else { return nil }
+        return Self.container?.appendingPathComponent(artworkName)
+    }
+
+    static var preview: PlayerWidgetState {
+        var value = Self()
+        value.title = "Your favorite music"
+        value.artist = "SpotiFLAC"
+        value.album = "Now Playing"
+        value.canPlay = true
+        value.canPrevious = true
+        value.canNext = true
+        return value
+    }
+}