feat(clients): event sound effects + optional text-to-speech

Add audible cues and optional spoken announcements for session events
(join/leave, channel + PM sent/recv, login, logout/connection-lost,
mic on/off, voice-activity, PTT) across all three clients, driven off
the shared C ABI vc_event stream so the mapping stays consistent.

TTS is off by default; when enabled it announces events and reads
message/PM bodies aloud. Master toggles + a sound-volume slider; the
per-utterance voice-activity and PTT cues default off. WAVs ship from
assets/sounds/.

Windows (built + verified): new VoiceCat.App/Notifications/ layer
(FeedbackSettings -> %AppData%\VoiceCat\feedback.json, SoundPlayerPool
via System.Media.SoundPlayer, SpeechAnnouncer via Prismatoid 0.3.0,
EventFeedback dispatcher); MainForm hooks; NotificationSettingsForm
under Settings > Notifications; csproj adds the Prismatoid PackageRef
and copies the WAVs into sounds\.

macOS + iOS (written, not yet built -- needs a Mac): shared
VoiceCatCore/Feedback/ (SoundEvent, EventFeedback = AVAudioPlayer pool
+ native AVSpeechSynthesizer, FeedbackSettings over UserDefaults); WAVs
bundled via Package.swift resources (.process). Hooks in SessionState/
AppState (iOS) and MainWindowController (macOS); settings UI in
SettingsView (iOS) and SettingsWindowController (macOS).

No core/server code touched; ctest --preset dev unaffected.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

clients/windows/VoiceCat.App/Notifications/SoundEvent.cs

@@ -0,0 +1,47 @@
+namespace VoiceCat.App.Notifications;
+
+/// <summary>
+/// The set of audible event cues. Each maps to a WAV in the app's <c>sounds\</c> folder
+/// (copied from <c>assets/sounds/</c> at build time). The same logical set is mirrored in the
+/// macOS/iOS clients so feedback stays consistent across platforms.
+/// </summary>
+public enum SoundEvent
+{
+    ChannelJoin,    // another user joined my channel
+    ChannelLeave,   // another user left my channel
+    ChannelRecv,    // channel text message from someone else
+    ChannelSent,    // channel text message I sent
+    PmRecv,         // private message received
+    PmSent,         // private message I sent
+    Login,          // connected / authenticated
+    Logout,         // clean disconnect
+    ConnectionLost, // unexpected disconnect
+    VoiceOn,        // my microphone stream started
+    VoiceOff,       // my microphone stream stopped
+    VaStart,        // my voice-activity began (off by default)
+    VaStop,         // my voice-activity ended (off by default)
+    Ptt,            // push-to-talk engaged (off by default)
+}
+
+internal static class SoundEventExtensions
+{
+    /// <summary>WAV file name (without directory) for each event, matching assets/sounds/.</summary>
+    public static string FileName(this SoundEvent ev) => ev switch
+    {
+        SoundEvent.ChannelJoin => "channel_join.wav",
+        SoundEvent.ChannelLeave => "channel_leave.wav",
+        SoundEvent.ChannelRecv => "channel_recv.wav",
+        SoundEvent.ChannelSent => "channel_sent.wav",
+        SoundEvent.PmRecv => "pm_recv.wav",
+        SoundEvent.PmSent => "pm_sent.wav",
+        SoundEvent.Login => "login.wav",
+        SoundEvent.Logout => "logout.wav",
+        SoundEvent.ConnectionLost => "connection_lost.wav",
+        SoundEvent.VoiceOn => "voice_on.wav",
+        SoundEvent.VoiceOff => "voice_off.wav",
+        SoundEvent.VaStart => "va_start.wav",
+        SoundEvent.VaStop => "va_stop.wav",
+        SoundEvent.Ptt => "ptt.wav",
+        _ => throw new ArgumentOutOfRangeException(nameof(ev), ev, null),
+    };
+}