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>

CLAUDE.md

@@ -29,9 +29,11 @@ cmake --preset dev
 cmake --build --preset dev
 
 # Run the tests (21 behavior tests — grows per milestone)
+# NOTE on Windows: run ctest via PowerShell, NOT Git Bash — MinGW binaries fail in Git Bash
+# with exit 0xc0000139 (STATUS_ENTRYPOINT_NOT_FOUND). PowerShell runs them correctly.
 ctest --preset dev                      # or: ctest --test-dir build/dev --output-on-failure
 
-# Run the binaries (Windows adds .exe; Linux/macOS no extension)
+# Run the binaries — same Windows rule: use PowerShell, not Git Bash
 ./build/dev/bin/vccli                    # headless test client
 ./build/dev/bin/voicecat-server --help
 ./build/dev/bin/voicecat-server --name "My Server"