Add a relay ping and backend status to the iOS app

The relay now answers a stateless ping with pong. The Relay card auto-checks on appear and address changes, showing latency or the connection error; this verifies the endpoint speaks the share protocol, not just that a TCP port is open.

apps/ios/Share/AppModel.swift

@@ -21,12 +21,16 @@ final class AppModel: ObservableObject {
   @Published var errorMessage: String?
   @Published var notice: String?
   @Published var showAdvanced = false
+  @Published var relayCheck: RelayCheck?
+  @Published var isCheckingRelay = false
 
   private let session: ShareSession = ShareNewSession()!
   private let receivedDir: URL
   private let seedsDir: URL
   private var pollTimer: Timer?
   private var noticeTask: Task<Void, Never>?
+  private var relayCheckTask: Task<Void, Never>?
+  private var relayCheckGeneration = 0
 
   private enum Keys {
     static let relay = "share.relay"
@@ -211,6 +215,63 @@ final class AppModel: ObservableObject {
     code = ShareGenerateCode()
   }
 
+  // MARK: - Relay reachability
+
+  /// Debounced automatic check used while the Relay card is visible. The
+  /// short delay keeps every keystroke in the relay field from opening a
+  /// socket.
+  func scheduleRelayCheck() {
+    relayCheckTask?.cancel()
+    relayCheckGeneration += 1
+    let generation = relayCheckGeneration
+    relayCheck = nil
+
+    relayCheckTask = Task { [weak self] in
+      try? await Task.sleep(nanoseconds: 350_000_000)
+      guard !Task.isCancelled, let self else { return }
+      await self.performRelayCheck(generation: generation)
+    }
+  }
+
+  /// Immediate check, used by the refresh button.
+  func checkRelay() {
+    relayCheckTask?.cancel()
+    relayCheckGeneration += 1
+    let generation = relayCheckGeneration
+    relayCheckTask = Task { [weak self] in
+      guard let self else { return }
+      await self.performRelayCheck(generation: generation)
+    }
+  }
+
+  private func performRelayCheck(generation: Int) async {
+    isCheckingRelay = true
+    let relay = relay
+
+    // ShareCheckRelay performs a dial plus the protocol ping/pong. Keep
+    // that off the main thread; the Go function has its own 5s timeouts.
+    let json = await withCheckedContinuation { (continuation: CheckedContinuation<String, Never>) in
+      DispatchQueue.global(qos: .utility).async {
+        continuation.resume(returning: ShareCheckRelay(relay))
+      }
+    }
+
+    guard generation == relayCheckGeneration else { return }
+    isCheckingRelay = false
+
+    guard let data = json.data(using: .utf8),
+          let decoded = try? JSONDecoder().decode(RelayCheck.self, from: data) else {
+      relayCheck = RelayCheck(
+        ok: false,
+        relay: relay.isEmpty ? ShareRelayDefault() : relay,
+        latencyMs: 0,
+        error: "Could not read the relay status."
+      )
+      return
+    }
+    relayCheck = decoded
+  }
+
   // MARK: - Status polling
 
   private func startPolling() {