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>

core/src/voicecat.cpp

@@ -117,6 +117,11 @@ vc_result vc_set_self_mute(vc_client* c, int mic_muted, int deafened) {
     return c->set_self_mute(mic_muted != 0, deafened != 0);
 }
 
+vc_result vc_set_output_volume(vc_client* c, float gain) {
+    if (c == nullptr) return VC_ERR_INVALID_ARG;
+    return c->set_output_volume(gain);
+}
+
 vc_result vc_set_remote_stream(vc_client* c, uint32_t user_id, uint32_t stream_id, float gain,
                                int muted, int noise_reduction) {
     if (c == nullptr) return VC_ERR_INVALID_ARG;