feat(sdk): add Swift and Kotlin mobile client foundations with push token proto

Swift SDK: Swift Package wrapping libquicproquo_ffi with QpqClient class
(connect, login, send, receive, disconnect) for iOS 15+ / macOS 13+.

Kotlin SDK: JNI bridge to libquicproquo_ffi with QpqClient class for
Android (aarch64, armv7) and JVM, Gradle build configuration.

Adds RegisterPushToken RPC (method ID 710) to device.proto for
APNs/FCM/WebPush device push token registration.

proto/qpq/v1/device.proto

@@ -32,3 +32,24 @@ message RevokeDeviceRequest {
 message RevokeDeviceResponse {
   bool success = 1;
 }
+
+// Push notification token registration.
+// Method ID: 710.
+// Clients call this after login to register their device for push notifications.
+
+enum PushPlatform {
+  PUSH_PLATFORM_UNSPECIFIED = 0;
+  PUSH_PLATFORM_APNS = 1;        // Apple Push Notification Service (iOS)
+  PUSH_PLATFORM_FCM = 2;         // Firebase Cloud Messaging (Android)
+  PUSH_PLATFORM_WEB_PUSH = 3;    // Web Push (browsers)
+}
+
+message RegisterPushTokenRequest {
+  bytes device_id = 1;
+  PushPlatform platform = 2;
+  string token = 3;              // Platform-specific push token
+}
+
+message RegisterPushTokenResponse {
+  bool success = 1;
+}