feat(ios): ship iOS SwiftUI client (VoiceCatiOS)

Full SwiftUI app at clients/apple/iOS/VoiceCatiOS.xcodeproj:
- 24 Swift source files: AppState + SessionState (@Observable @MainActor),
  AudioSessionManager (AVAudioSession owner + interruption/route handling),
  ServerListStore/SavedServer (App Group container + Keychain sharing),
  and 14 SwiftUI views covering the full feature set
- NavigationSplitView on iPad, TabView on iPhone (horizontalSizeClass)
- Channel tree via OutlineGroup, user list with context menu admin actions
- PTT via DragGesture(minimumDistance: 0) + @GestureState
- onEvent closures hop to MainActor via Task { @MainActor in ... }
- App Group: group.cat.voice.VoiceCat (shared with future ReplayKit extension)

C ABI: add vc_audio_suspend / vc_audio_resume (AudioEngine::suspend/resume)
called by AudioSessionManager on AVAudioSession interruption events.

XCFramework: add ios-arm64 and ios-arm64-simulator slices to build-xcframework.sh;
Package.swift gains .iOS(.v17) platform; CMakePresets.json adds apple-ios /
apple-ios-sim presets with arm64-ios / arm64-ios-simulator vcpkg triplets.

Verified: xcodebuild -target VoiceCatiOS -sdk iphonesimulator26.5 BUILD SUCCEEDED.

clients/apple/iOS/VoiceCatiOS/Views/PasswordPromptView.swift

@@ -0,0 +1,65 @@
+import SwiftUI
+
+struct PasswordPromptView: View {
+    @Environment(AppState.self) private var appState
+    @Environment(\.dismiss) private var dismiss
+
+    @State private var username = ""
+    @State private var password = ""
+
+    var body: some View {
+        NavigationStack {
+            Form {
+                Section {
+                    if let server = appState.connectingServer {
+                        Text("Connecting to \(server.displayString)")
+                            .font(.caption)
+                            .foregroundStyle(.secondary)
+                    }
+                    if appState.connectingServer?.authMode == .password {
+                        TextField("Username", text: $username)
+                            .textContentType(.username)
+                            .autocorrectionDisabled()
+                            .textInputAutocapitalization(.never)
+                            .accessibilityLabel("Username")
+                    }
+                    SecureField("Password", text: $password)
+                        .textContentType(.password)
+                        .accessibilityLabel("Password")
+                }
+
+                if !appState.connectStatus.isEmpty && appState.connectStatus.lowercased().contains("failed") {
+                    Section {
+                        Text(appState.connectStatus)
+                            .foregroundStyle(.red)
+                            .accessibilityLabel("Error: \(appState.connectStatus)")
+                    }
+                }
+            }
+            .navigationTitle("Sign In")
+            .navigationBarTitleDisplayMode(.inline)
+            .toolbar {
+                ToolbarItem(placement: .cancellationAction) {
+                    Button("Cancel") {
+                        dismiss()
+                        appState.cancelConnect()
+                    }
+                }
+                ToolbarItem(placement: .confirmationAction) {
+                    Button("Connect") {
+                        dismiss()
+                        let uname = appState.connectingServer?.savedUsername.isEmpty == false
+                            ? appState.connectingServer!.savedUsername
+                            : username
+                        appState.authenticateUser(username: uname, password: password)
+                    }
+                    .disabled(password.isEmpty)
+                }
+            }
+        }
+        .onAppear {
+            username = appState.connectingServer?.savedUsername ?? ""
+        }
+        .interactiveDismissDisabled()
+    }
+}