feat(macos): per-app audio selection for screen sharing

Let users choose what the SCREEN_AUDIO stream captures before sharing:
share everything, only selected apps, or all except selected apps, plus a
first-class "Exclude screen reader (VoiceOver) audio" toggle.

ScreenCaptureKit filters audio per application, so ScreenAudioCapture now
takes a ScreenAudioSelection and builds the matching SCContentFilter
(including:/excludingApplications:). New ScreenSharePickerSheet lists
running apps from SCShareableContent. iOS left untouched -- ReplayKit only
delivers the mixed system stream, so per-app filtering is impossible there.

clients/apple/macOS/VoiceCatMac/Windows/MainWindowController.swift

@@ -31,6 +31,8 @@ final class MainWindowController: NSWindowController, NSWindowDelegate {
     internal var micStreamId: UInt32 = 0
     private var screenStreamId: UInt32 = 0
     private var screenCapture: ScreenAudioCapture?
+    // Last app/exclusion choice from the share picker; reused as the default next time.
+    private var screenAudioSelection: ScreenAudioSelection = .default
     internal var pttKeyCode: UInt16 = 0x60  // F8
     private var pttMonitor: Any?
     private var serverMuted = false
@@ -720,17 +722,15 @@ final class MainWindowController: NSWindowController, NSWindowDelegate {
 
     @objc private func screenAudioClicked() {
         if screenStreamId == 0 {
-            // Announce the stream now; ScreenCaptureKit capture starts once the server's
-            // StreamAnnounceResult lands (the .streamStarted event), when the effective audio
-            // config — and thus the channel count to capture — is known. See startScreenCapture.
-            let (result, streamId) = client.startStream(StreamDescriptor(kind: .screenAudio, deviceId: nil, label: "Desktop audio"))
-            if result == .ok {
-                screenStreamId = streamId
-                setShareScreenButton(active: true)
-                addActivity("Starting screen audio share…")
-            } else {
-                addActivity("Failed to start screen audio: \(result)")
+            // Let the user pick what to share (apps to include/exclude, screen-reader audio)
+            // before we announce anything. The picker remembers the previous choice.
+            let sheet = ScreenSharePickerSheet(selection: screenAudioSelection)
+            sheet.onComplete = { [weak self] selection in
+                guard let self, let selection else { return }  // nil = cancelled
+                self.screenAudioSelection = selection
+                self.beginScreenAudioShare()
             }
+            presentSheet(sheet)
         } else {
             stopScreenCapture()
             client.stopStream(screenStreamId)
@@ -740,6 +740,21 @@ final class MainWindowController: NSWindowController, NSWindowDelegate {
         }
     }
 
+    /// Announce the SCREEN_AUDIO stream with the chosen selection in hand. ScreenCaptureKit
+    /// capture starts once the server's StreamAnnounceResult lands (the .streamStarted event),
+    /// when the effective audio config — and thus the channel count — is known. See
+    /// startScreenCapture.
+    private func beginScreenAudioShare() {
+        let (result, streamId) = client.startStream(StreamDescriptor(kind: .screenAudio, deviceId: nil, label: "Desktop audio"))
+        if result == .ok {
+            screenStreamId = streamId
+            setShareScreenButton(active: true)
+            addActivity("Starting screen audio share…")
+        } else {
+            addActivity("Failed to start screen audio: \(result)")
+        }
+    }
+
     private func setShareScreenButton(active: Bool) {
         shareScreenButton?.title = active ? "Stop Screen Audio" : "Share Screen Audio"
         shareScreenButton?.image = NSImage(
@@ -760,7 +775,7 @@ final class MainWindowController: NSWindowController, NSWindowDelegate {
         let channels: UInt32 = (cfgResult == .ok && cfg?.stereo == true) ? 2 : 1
 
         let streamId = screenStreamId
-        let capture = ScreenAudioCapture(channels: channels) { [weak self] pcm, samples, ch in
+        let capture = ScreenAudioCapture(channels: channels, selection: screenAudioSelection) { [weak self] pcm, samples, ch in
             self?.client.feedPcm(streamId: streamId, pcm: pcm, samplesPerChannel: samples, channels: ch)
         }
         screenCapture = capture
@@ -768,7 +783,8 @@ final class MainWindowController: NSWindowController, NSWindowDelegate {
         Task { @MainActor in
             do {
                 try await capture.start()
-                addActivity("Started sharing screen audio (\(channels == 2 ? "stereo" : "mono"))")
+                addActivity("Started sharing screen audio (\(channels == 2 ? "stereo" : "mono"))"
+                            + "\(Self.scopeSuffix(for: screenAudioSelection))")
             } catch {
                 // Most commonly: Screen Recording permission denied. Roll back the stream.
                 screenCapture = nil
@@ -788,6 +804,21 @@ final class MainWindowController: NSWindowController, NSWindowDelegate {
         screenCapture = nil
     }
 
+    /// A short human-readable description of the share scope for the activity log.
+    private static func scopeSuffix(for selection: ScreenAudioSelection) -> String {
+        var parts: [String] = []
+        switch selection.scope {
+        case .entireDesktop: break
+        case .onlyApps(let ids):  if !ids.isEmpty { parts.append("only \(ids.count) app(s)") }
+        case .allExcept(let ids): if !ids.isEmpty { parts.append("excluding \(ids.count) app(s)") }
+        }
+        // For .onlyApps the screen reader is already excluded, so don't claim it twice.
+        if selection.excludeScreenReader {
+            if case .onlyApps = selection.scope {} else { parts.append("no screen reader") }
+        }
+        return parts.isEmpty ? "" : " — " + parts.joined(separator: ", ")
+    }
+
     @objc private func muteChanged() {
         let muted = muteButton?.state == .on
         let deafened = deafenButton?.state == .on