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.

sdks/swift/Sources/QuicProQuo/QpqError.swift

@@ -0,0 +1,23 @@
+/// Errors returned by the QuicProQuo SDK.
+public enum QpqError: Error, Sendable, CustomStringConvertible {
+    /// Connection to the server failed.
+    case connectionFailed(String)
+    /// OPAQUE authentication failed (bad credentials).
+    case authFailed(String)
+    /// The operation timed out.
+    case timeout(String)
+    /// The client is not connected.
+    case notConnected
+    /// A generic error from the FFI layer.
+    case ffiError(String)
+
+    public var description: String {
+        switch self {
+        case .connectionFailed(let msg): return "connection failed: \(msg)"
+        case .authFailed(let msg): return "auth failed: \(msg)"
+        case .timeout(let msg): return "timeout: \(msg)"
+        case .notConnected: return "not connected"
+        case .ffiError(let msg): return "FFI error: \(msg)"
+        }
+    }
+}