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/BanUserSheet.swift

@@ -0,0 +1,93 @@
+import AppKit
+
+final class BanUserSheet: NSViewController {
+
+    var onComplete: ((String?, UInt64) -> Void)?
+
+    private let targetNickname: String
+    private let reasonField = NSTextField()
+    private let durationPicker = NSSegmentedControl(
+        labels: ["1 hour", "24 hours", "7 days", "Permanent"],
+        trackingMode: .selectOne,
+        target: nil, action: nil
+    )
+
+    init(nickname: String) {
+        self.targetNickname = nickname
+        super.init(nibName: nil, bundle: nil)
+    }
+
+    required init?(coder: NSCoder) { fatalError() }
+
+    override func loadView() {
+        view = NSView(frame: NSRect(x: 0, y: 0, width: 360, height: 160))
+    }
+
+    override func viewDidLoad() {
+        super.viewDidLoad()
+        buildUI()
+    }
+
+    private func buildUI() {
+        let titleLabel = NSTextField(labelWithString: "Ban \(targetNickname)")
+        titleLabel.font = .boldSystemFont(ofSize: 13)
+
+        let reasonLabel = NSTextField(labelWithString: "Reason:")
+        reasonField.placeholderString = "Ban reason (optional)"
+        reasonField.setAccessibilityLabel("Ban reason")
+
+        let durationLabel = NSTextField(labelWithString: "Duration:")
+        durationPicker.selectedSegment = 3
+        durationPicker.setAccessibilityLabel("Ban duration")
+
+        let cancelButton = NSButton(title: "Cancel", target: self, action: #selector(cancelClicked))
+        cancelButton.bezelStyle = .rounded
+
+        let banButton = NSButton(title: "Ban", target: self, action: #selector(banClicked))
+        banButton.bezelStyle = .rounded; banButton.keyEquivalent = "\r"
+        banButton.setAccessibilityLabel("Confirm ban")
+
+        let buttonRow = NSStackView(views: [NSView(), cancelButton, banButton])
+        buttonRow.orientation = .horizontal; buttonRow.spacing = 8
+
+        let grid = NSGridView(views: [
+            [reasonLabel, reasonField],
+            [durationLabel, durationPicker],
+        ])
+        grid.rowSpacing = 8; grid.columnSpacing = 8
+        grid.column(at: 0).xPlacement = .trailing
+
+        let stack = NSStackView(views: [titleLabel, grid, buttonRow])
+        stack.orientation = .vertical; stack.spacing = 12
+        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),
+            stack.bottomAnchor.constraint(equalTo: view.bottomAnchor),
+        ])
+    }
+
+    @objc private func banClicked() {
+        let reason = reasonField.stringValue.trimmingCharacters(in: .whitespaces)
+        let expiresUnixMs = expiryFromSelection()
+        dismiss(nil)
+        onComplete?(reason.isEmpty ? nil : reason, expiresUnixMs)
+    }
+
+    @objc private func cancelClicked() {
+        dismiss(nil)
+    }
+
+    private func expiryFromSelection() -> UInt64 {
+        let nowMs = UInt64(Date().timeIntervalSince1970 * 1000)
+        switch durationPicker.selectedSegment {
+        case 0: return nowMs + 3_600_000          // 1 hour
+        case 1: return nowMs + 86_400_000         // 24 hours
+        case 2: return nowMs + 604_800_000        // 7 days
+        default: return 0                          // permanent (0 = no expiry)
+        }
+    }
+}