fix: use Task { @mainactor in } in all view button actions to prevent SIGSEGV crash (#121)

Bare Task { } closures inside SwiftUI Button actions caused an
EXC_BAD_ACCESS (SIGSEGV) crash on macOS 26 when the Swift concurrency
runtime called MainActor.assumeIsolated() on a stale executor reference.

This was reliably triggered by:
1. Replug a microphone while recording (causes AVAudioEngineConfigurationChange)
2. App auto-recovers via onAudioDeviceChanged → sets appStatus = .idle
3. User clicks Stop Recording button on the now-stale SwiftUI view node
4. _ButtonGesture dispatches via MainActor.assumeIsolated() → reads freed memory → crash

Fix: annotate every Task spawned from a Button/onAppear in views with
@mainactor, which removes the implicit assumeIsolated() call and binds
the task directly to the main actor instead.

Also reset silenceCallbackFired and maxDurationCallbackFired in
AudioEngine.handleAudioConfigurationChange so stale callback state
cannot fire spurious stop/timeout events after a device reconnect.

Author: jatinkrmalik

Sources/VocaMac/Services/AudioEngine.swift

@@ -88,6 +88,8 @@ final class AudioEngine {
             VocaLogger.warning(.audioEngine, "Configuration changed while recording — forcing stop and reset")
             // Tear down the stale recording state
             isCurrentlyRecording = false
+            silenceCallbackFired = false
+            maxDurationCallbackFired = false
             engine.inputNode.removeTap(onBus: 0)
             engine.stop()
         }

Sources/VocaMac/Views/MenuBarView.swift

@@ -219,7 +219,7 @@ struct MenuBarView: View {
                 // Stop/recovery button — visible during recording so the user
                 // can unstick the app if the hotkey isn't responding
                 Button {
-                    Task {
+                    Task { @MainActor in
                         await appState.stopRecordingAndTranscribe()
                     }
                 } label: {

Sources/VocaMac/Views/OnboardingView.swift

@@ -160,7 +160,7 @@ struct OnboardingView: View {
         }
         .frame(width: 600, height: 550)
         .onAppear {
-            Task {
+            Task { @MainActor in
                 await appState.performStartup()
             }
         }
@@ -425,12 +425,12 @@ struct ModelSelectionStep: View {
                                 return recommended == prefix || recommended.hasPrefix(prefix + "-")
                             }(),
                             onSelect: {
-                                Task {
+                                Task { @MainActor in
                                     await appState.loadModel(modelInfo.size)
                                 }
                             },
                             onDownload: {
-                                Task {
+                                Task { @MainActor in
                                     await appState.downloadModel(modelInfo.size)
                                 }
                             }
@@ -785,12 +785,12 @@ struct QuickTestStep: View {
 
     private func toggleRecording() {
         if isRecording {
-            Task {
+            Task { @MainActor in
                 await appState.stopRecordingAndTranscribe()
                 isRecording = false
             }
         } else {
-            Task {
+            Task { @MainActor in
                 await appState.startRecording()
                 isRecording = true
             }

Sources/VocaMac/Views/SettingsView.swift

@@ -490,13 +490,13 @@ struct ModelRow: View {
                 EmptyView()
             } else if model.isDownloaded {
                 Button("Load") {
-                    Task { await appState.loadModel(model.size) }
+                    Task { @MainActor in await appState.loadModel(model.size) }
                 }
                 .controlSize(.small)
                 .buttonStyle(.borderedProminent)
             } else {
                 Button("Download & Load") {
-                    Task {
+                    Task { @MainActor in
                         await appState.downloadModel(model.size)
                         if appState.availableModels.first(where: { $0.size == model.size })?.isDownloaded == true {
                             await appState.loadModel(model.size)
@@ -511,7 +511,7 @@ struct ModelRow: View {
         .alert("Use Unoptimized Model?", isPresented: $showForceDownloadAlert) {
             Button("Cancel", role: .cancel) {}
             Button(model.isDownloaded ? "Load Anyway" : "Download & Load", role: .destructive) {
-                Task {
+                Task { @MainActor in
                     if !model.isDownloaded {
                         await appState.downloadModel(model.size)
                     }
@@ -673,7 +673,7 @@ struct AboutTab: View {
                 .foregroundStyle(.tertiary)
 
             Button {
-                Task {
+                Task { @MainActor in
                     await appState.updateChecker.checkForUpdates()
                     if case .updateAvailable(let info) = appState.updateChecker.updateState {
                         updateInfoForSheet = info

Sources/VocaMac/Views/UpdateView.swift

@@ -103,7 +103,7 @@ struct UpdateDetailView: View {
                 Spacer()
 
                 Button("Download & Install") {
-                    Task {
+                    Task { @MainActor in
                         await appState.updateChecker.downloadUpdate(info)
                     }
                 }
@@ -168,7 +168,7 @@ struct UpdateDetailView: View {
                     .buttonStyle(.bordered)
 
                     Button("Retry") {
-                        Task {
+                        Task { @MainActor in
                             await appState.updateChecker.downloadUpdate(info)
                         }
                     }