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.App/Forms/PrivateMessageForm.cs

@@ -0,0 +1,115 @@
+using VoiceCat.Interop;
+
+namespace VoiceCat.App.Forms;
+
+/// <summary>
+/// Modeless window for a single private message conversation.
+/// Created by MainForm and shown non-modally; MainForm routes incoming PMs here.
+/// </summary>
+public sealed class PrivateMessageForm : Form
+{
+    private readonly VoiceCatClient _client;
+    private readonly uint _userId;
+    private readonly uint _selfUserId;
+
+    private readonly RichTextBox _rtbHistory;
+    private readonly TextBox _txtCompose;
+    private readonly Button _btnSend;
+
+    public PrivateMessageForm(VoiceCatClient client, uint userId, string nickname, uint selfUserId)
+    {
+        _client = client;
+        _userId = userId;
+        _selfUserId = selfUserId;
+
+        Text = $"Private Message — {nickname}";
+        ClientSize = new Size(480, 360);
+        MinimumSize = new Size(320, 240);
+        StartPosition = FormStartPosition.Manual;
+
+        _rtbHistory = new RichTextBox
+        {
+            ReadOnly = true,
+            Dock = DockStyle.Fill,
+            ScrollBars = RichTextBoxScrollBars.Vertical,
+            BackColor = SystemColors.Window,
+            AccessibleName = "Message history",
+        };
+
+        var composePanel = new TableLayoutPanel
+        {
+            Dock = DockStyle.Bottom,
+            Height = 34,
+            ColumnCount = 2,
+            RowCount = 1,
+            Padding = new Padding(0, 3, 0, 0),
+        };
+        composePanel.ColumnStyles.Add(new ColumnStyle(SizeType.Percent, 100F));
+        composePanel.ColumnStyles.Add(new ColumnStyle(SizeType.Absolute, 68F));
+        composePanel.RowStyles.Add(new RowStyle(SizeType.Percent, 100F));
+
+        _txtCompose = new TextBox
+        {
+            Dock = DockStyle.Fill,
+            AccessibleName = "Message text",
+        };
+        _btnSend = new Button
+        {
+            Text = "&Send",
+            Dock = DockStyle.Fill,
+        };
+
+        composePanel.Controls.Add(_txtCompose, 0, 0);
+        composePanel.Controls.Add(_btnSend, 1, 0);
+
+        Controls.Add(_rtbHistory);
+        Controls.Add(composePanel);
+
+        _txtCompose.KeyDown += (_, e) =>
+        {
+            if (e.KeyCode == Keys.Enter) { Send(); e.Handled = e.SuppressKeyPress = true; }
+        };
+        _btnSend.Click += (_, _) => Send();
+    }
+
+    // Called by MainForm when a PM message arrives (or we send one and the server echoes it).
+    public void AppendMessage(string time, bool isSelf, string sender, string text)
+    {
+        if (IsDisposed) return;
+        string line = $"[{time}] {sender}: {text}\n";
+        int selStart = _rtbHistory.TextLength;
+        _rtbHistory.AppendText(line);
+        if (isSelf)
+        {
+            _rtbHistory.Select(selStart, line.Length);
+            _rtbHistory.SelectionColor = Color.Gray;
+            _rtbHistory.Select(_rtbHistory.TextLength, 0);
+            _rtbHistory.SelectionColor = _rtbHistory.ForeColor;
+        }
+        _rtbHistory.ScrollToCaret();
+    }
+
+    // Called by MainForm for status events (e.g., the other user disconnected).
+    public void AppendActivity(string text)
+    {
+        if (IsDisposed) return;
+        string line = $"[{DateTime.Now:HH:mm}] {text}\n";
+        int selStart = _rtbHistory.TextLength;
+        _rtbHistory.AppendText(line);
+        _rtbHistory.Select(selStart, line.Length);
+        _rtbHistory.SelectionColor = Color.Gray;
+        _rtbHistory.Select(_rtbHistory.TextLength, 0);
+        _rtbHistory.SelectionColor = _rtbHistory.ForeColor;
+        _rtbHistory.ScrollToCaret();
+    }
+
+    private void Send()
+    {
+        string msg = _txtCompose.Text.Trim();
+        if (string.IsNullOrEmpty(msg)) return;
+        _client.SendText(VcTextScope.Private, _userId, msg);
+        _txtCompose.Clear();
+        // No optimistic echo: the server relays our own message back, so AppendMessage is
+        // called from MainForm.HandleTextMessage when the echo arrives.
+    }
+}