feat(windows): UI overhaul -- toolbar, unified log, PM windows, channel counts, output volume

- Voice actions (Join Voice, Share Screen Audio) moved to a ToolStrip toolbar and
  a new Voice menu in the menu bar; removed from the bottom voice panel
- Activity log and chat log collapsed into a single RichTextBox (rtbLog); activity
  events appear in gray, chat messages in default color
- Private messaging reworked: each conversation opens in its own modeless
  PrivateMessageForm instead of sharing the main chat log via a scope dropdown;
  cboScope removed; main compose bar always sends to the current channel
- New "Messages -> New Private Message..." menu item (Ctrl+P) opens a UserPickerDialog
  listing all connected server users (not just the current channel) so you can PM
  anyone on the server
- Channel tree now shows live user counts, e.g. "General (3)" -- counts sourced from
  the existing _users dictionary which already tracks all server users with channel IDs
- Global output volume slider (TrackBar, 0-100, default 80) added to the right panel;
  wired to new vc_set_output_volume C ABI function that applies a master gain multiplier
  in the audio engine playback callback after mixing all streams
- vc_set_output_volume added end-to-end: voicecat.h, audio_engine.h/.cpp,
  client.h/.cpp, voicecat.cpp, NativeMethods.cs, VoiceCatClient.cs
- Documented Windows PowerShell ctest requirement in AGENTS.md and CLAUDE.md:
  MinGW binaries exit 0xc0000139 in Git Bash; always run ctest/.exe via PowerShell

22/22 ctest green (PowerShell); dotnet build 0 warnings.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

clients/windows/VoiceCat.Interop/NativeMethods.cs

@@ -77,6 +77,9 @@ internal static partial class NativeMethods
     [LibraryImport(LibName)]
     internal static partial VcResult vc_set_self_mute(nint c, int micMuted, int deafened);
 
+    [LibraryImport(LibName)]
+    internal static partial VcResult vc_set_output_volume(nint c, float gain);
+
     [LibraryImport(LibName)]
     internal static partial VcResult vc_set_remote_stream(nint c, uint userId, uint streamId,
         float gain, int muted, int noiseReduction);

clients/windows/VoiceCat.Interop/VoiceCatClient.cs

@@ -204,6 +204,9 @@ public sealed class VoiceCatClient : IDisposable
     public VcResult SetSelfMute(bool micMuted, bool deafened) =>
         NativeMethods.vc_set_self_mute(_handle.DangerousGetHandle(), micMuted ? 1 : 0, deafened ? 1 : 0);
 
+    public VcResult SetOutputVolume(float gain) =>
+        NativeMethods.vc_set_output_volume(_handle.DangerousGetHandle(), gain < 0f ? 0f : gain);
+
     public VcResult SetRemoteStream(uint userId, uint streamId, float gain, bool muted, bool noiseReduction) =>
         NativeMethods.vc_set_remote_stream(_handle.DangerousGetHandle(), userId, streamId, gain,
             muted ? 1 : 0, noiseReduction ? 1 : 0);