feat(macos): VoiceCatMac AppKit client + fix xcodebuild

The previous "shipped" claim was false — xcodebuild had never been run and
the macOS app source was never committed. This commit adds the 17 Swift
source files + xcodeproj and fixes three real defect classes so Debug and
Release both build clean:

1. MainWindowController.swift compile errors:
   - NSAccessibility.post arg order (element:notification:userInfo:)
   - NSAccessibilityPriorityMedium -> NSAccessibilityPriorityLevel.medium
   - StreamSummary.streamId -> .id (Identifiable conformance)
   - drop redundant VoiceCatResult.description extension
2. Linker: add -lc++ to OTHER_LDFLAGS (libvoicecat-fat.a is C++20; pure-Swift
   app target has no .cpp sources so libc++ wasn't pulled in — swift test
   passed because Package.swift testTarget has linkerSettings: c++).
3. Release config: add ONLY_ACTIVE_ARCH=YES (XCFramework only has arm64).

Verified: clean Debug + Release builds, otool -L shows libc++.1.dylib,
nm shows _vc_client_create/_vc_version_string, app launches and runs.

clients/apple/macOS/VoiceCatMac/Sheets/PerUserTuningSheet.swift

@@ -0,0 +1,123 @@
+import AppKit
+import VoiceCatCore
+
+final class PerUserTuningSheet: NSViewController {
+
+    private let client: VoiceCatClient
+    private let userId: UInt32
+    private let nickname: String
+
+    private var streams: [StreamSummary] = []
+    private var rows: [StreamRow] = []
+
+    private struct StreamRow {
+        let streamId: UInt32
+        let label: String
+        let gainSlider: NSSlider
+        let muteCheckbox: NSButton
+        let nrCheckbox: NSButton
+    }
+
+    init(client: VoiceCatClient, userId: UInt32, nickname: String) {
+        self.client = client
+        self.userId = userId
+        self.nickname = nickname
+        super.init(nibName: nil, bundle: nil)
+    }
+
+    required init?(coder: NSCoder) { fatalError() }
+
+    override func loadView() {
+        view = NSView(frame: NSRect(x: 0, y: 0, width: 400, height: 200))
+    }
+
+    override func viewDidLoad() {
+        super.viewDidLoad()
+        streams = client.listUserStreams(userId)
+        buildUI()
+    }
+
+    private func buildUI() {
+        let titleLabel = NSTextField(labelWithString: "Volume & Noise Settings — \(nickname)")
+        titleLabel.font = .boldSystemFont(ofSize: 13)
+
+        var rowViews: [NSView] = [titleLabel]
+
+        for stream in streams {
+            let (_, state) = client.getRemoteStream(userId: userId, streamId: stream.id)
+            let gain = state?.gain ?? 1.0
+            let muted = state?.muted ?? false
+            let nr = state?.noiseReduction ?? false
+
+            let gainSlider = NSSlider(value: Double(gain * 100), minValue: 0, maxValue: 200, target: self, action: #selector(sliderChanged))
+            gainSlider.tag = Int(stream.id)
+            gainSlider.numberOfTickMarks = 0
+            gainSlider.setAccessibilityLabel("Volume for \(stream.label): \(Int(gain * 100)) percent")
+
+            let muteCheckbox = NSButton(checkboxWithTitle: "Mute", target: self, action: #selector(muteChanged))
+            muteCheckbox.state = muted ? .on : .off
+            muteCheckbox.tag = Int(stream.id)
+            muteCheckbox.setAccessibilityLabel("Mute \(stream.label)")
+
+            let nrCheckbox = NSButton(checkboxWithTitle: "Noise reduction", target: self, action: #selector(nrChanged))
+            nrCheckbox.state = nr ? .on : .off
+            nrCheckbox.tag = Int(stream.id)
+            nrCheckbox.setAccessibilityLabel("Noise reduction for \(stream.label)")
+
+            let streamLabel = NSTextField(labelWithString: "\(stream.label):")
+            let gainLabel = NSTextField(labelWithString: "Volume:")
+            let row = NSStackView(views: [streamLabel, gainLabel, gainSlider, muteCheckbox, nrCheckbox])
+            row.orientation = .horizontal; row.spacing = 8
+            rowViews.append(row)
+
+            rows.append(StreamRow(streamId: stream.id, label: stream.label,
+                                  gainSlider: gainSlider, muteCheckbox: muteCheckbox,
+                                  nrCheckbox: nrCheckbox))
+        }
+
+        if streams.isEmpty {
+            rowViews.append(NSTextField(labelWithString: "This user has no active streams."))
+        }
+
+        let doneButton = NSButton(title: "Done", target: self, action: #selector(doneClicked))
+        doneButton.bezelStyle = .rounded; doneButton.keyEquivalent = "\r"
+        doneButton.setAccessibilityLabel("Close settings")
+        rowViews.append(NSStackView(views: [NSView(), doneButton]))
+
+        let stack = NSStackView(views: rowViews)
+        stack.orientation = .vertical
+        stack.spacing = 10
+        stack.edgeInsets = NSEdgeInsets(top: 16, left: 16, bottom: 16, right: 16)
+        stack.translatesAutoresizingMaskIntoConstraints = false
+        view.addSubview(stack)
+        NSLayoutConstraint.activate([
+            stack.topAnchor.constraint(equalTo: view.topAnchor),
+            stack.leadingAnchor.constraint(equalTo: view.leadingAnchor),
+            stack.trailingAnchor.constraint(equalTo: view.trailingAnchor),
+        ])
+    }
+
+    @objc private func sliderChanged(_ sender: NSSlider) {
+        apply(streamId: UInt32(sender.tag))
+    }
+
+    @objc private func muteChanged(_ sender: NSButton) {
+        apply(streamId: UInt32(sender.tag))
+    }
+
+    @objc private func nrChanged(_ sender: NSButton) {
+        apply(streamId: UInt32(sender.tag))
+    }
+
+    private func apply(streamId: UInt32) {
+        guard let row = rows.first(where: { $0.streamId == streamId }) else { return }
+        let gain = Float(row.gainSlider.doubleValue) / 100.0
+        let muted = row.muteCheckbox.state == .on
+        let nr = row.nrCheckbox.state == .on
+        client.setRemoteStream(userId: userId, streamId: streamId,
+                               gain: gain, muted: muted, noiseReduction: nr)
+        row.gainSlider.setAccessibilityLabel("Volume for \(row.label): \(Int(row.gainSlider.doubleValue)) percent")
+    }
+
+    @objc private func doneClicked() { dismiss(nil) }
+}