fix: avoid iOS talk mode audio tap crash

apps/ios/Sources/RootCanvas.swift

@@ -253,7 +253,7 @@ private struct CanvasContent: View {
 private struct OverlayButton: View {
     let systemImage: String
     let brighten: Bool
-    var tint: Color? = nil
+    var tint: Color?
     var isActive: Bool = false
     let action: () -> Void
 

apps/ios/Sources/Voice/TalkModeManager.swift

@@ -7,6 +7,7 @@ import Speech
 @MainActor
 @Observable
 final class TalkModeManager: NSObject {
+    private typealias SpeechRequest = SFSpeechAudioBufferRecognitionRequest
     var isEnabled: Bool = false
     var isListening: Bool = false
     var isSpeaking: Bool = false
@@ -105,9 +106,8 @@ final class TalkModeManager: NSObject {
         let input = self.audioEngine.inputNode
         let format = input.outputFormat(forBus: 0)
         input.removeTap(onBus: 0)
-        input.installTap(onBus: 0, bufferSize: 2048, format: format) { [weak request] buffer, _ in
-            request?.append(buffer)
-        }
+        let tapBlock = Self.makeAudioTapAppendCallback(request: request)
+        input.installTap(onBus: 0, bufferSize: 2048, format: format, block: tapBlock)
 
         self.audioEngine.prepare()
         try self.audioEngine.start()
@@ -135,6 +135,12 @@ final class TalkModeManager: NSObject {
         self.speechRecognizer = nil
     }
 
+    private nonisolated static func makeAudioTapAppendCallback(request: SpeechRequest) -> AVAudioNodeTapBlock {
+        { buffer, _ in
+            request.append(buffer)
+        }
+    }
+
     private func handleTranscript(transcript: String, isFinal: Bool) async {
         let trimmed = transcript.trimmingCharacters(in: .whitespacesAndNewlines)
         if self.isSpeaking, self.interruptOnSpeech {