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/Info.plist

@@ -0,0 +1,28 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
+<plist version="1.0">
+<dict>
+	<key>CFBundleExecutable</key>
+	<string>$(EXECUTABLE_NAME)</string>
+	<key>CFBundleIdentifier</key>
+	<string>$(PRODUCT_BUNDLE_IDENTIFIER)</string>
+	<key>CFBundleInfoDictionaryVersion</key>
+	<string>6.0</string>
+	<key>CFBundleName</key>
+	<string>VoiceCat</string>
+	<key>CFBundlePackageType</key>
+	<string>APPL</string>
+	<key>CFBundleShortVersionString</key>
+	<string>0.0.1</string>
+	<key>CFBundleVersion</key>
+	<string>1</string>
+	<key>LSMinimumSystemVersion</key>
+	<string>14.0</string>
+	<key>NSHumanReadableCopyright</key>
+	<string>Copyright © 2026 VoiceCat contributors. All rights reserved.</string>
+	<key>NSMicrophoneUsageDescription</key>
+	<string>VoiceCat uses your microphone to transmit voice audio to other participants in the current channel.</string>
+	<key>NSPrincipalClass</key>
+	<string>NSApplication</string>
+</dict>
+</plist>