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/UserPickerDialog.cs

@@ -0,0 +1,99 @@
+using VoiceCat.Interop;
+
+namespace VoiceCat.App.Forms;
+
+/// <summary>
+/// Simple modal dialog for picking one user from a list of all server users.
+/// Used by "New Private Message" to let the user send a PM to anyone on the server.
+/// </summary>
+public sealed class UserPickerDialog : Form
+{
+    private readonly ListBox _lstUsers;
+    private readonly Button _btnOk;
+    private readonly Button _btnCancel;
+
+    private readonly List<UserInfo> _users;
+
+    public uint SelectedUserId { get; private set; }
+
+    public UserPickerDialog(List<UserInfo> users)
+    {
+        _users = users;
+
+        Text = "New Private Message";
+        ClientSize = new Size(280, 320);
+        MinimumSize = new Size(220, 240);
+        FormBorderStyle = FormBorderStyle.FixedDialog;
+        MaximizeBox = false;
+        MinimizeBox = false;
+        StartPosition = FormStartPosition.CenterParent;
+
+        var lbl = new Label
+        {
+            Text = "Select a user:",
+            AutoSize = true,
+            Padding = new Padding(6, 6, 6, 2),
+            Dock = DockStyle.Top,
+        };
+
+        _lstUsers = new ListBox
+        {
+            Dock = DockStyle.Fill,
+            AccessibleName = "User list",
+        };
+        foreach (var u in users)
+            _lstUsers.Items.Add(new UserItem(u.Id, u.Nickname));
+
+        var btnPanel = new FlowLayoutPanel
+        {
+            Dock = DockStyle.Bottom,
+            FlowDirection = FlowDirection.RightToLeft,
+            Height = 40,
+            Padding = new Padding(4),
+            AutoSize = false,
+        };
+
+        _btnCancel = new Button
+        {
+            Text = "Cancel",
+            DialogResult = DialogResult.Cancel,
+            AutoSize = true,
+        };
+        _btnOk = new Button
+        {
+            Text = "OK",
+            DialogResult = DialogResult.OK,
+            AutoSize = true,
+            Enabled = false,
+        };
+        AcceptButton = _btnOk;
+        CancelButton = _btnCancel;
+
+        btnPanel.Controls.Add(_btnCancel);
+        btnPanel.Controls.Add(_btnOk);
+
+        Controls.Add(_lstUsers);
+        Controls.Add(lbl);
+        Controls.Add(btnPanel);
+
+        _lstUsers.SelectedIndexChanged += (_, _) =>
+            _btnOk.Enabled = _lstUsers.SelectedItem is UserItem;
+        _lstUsers.DoubleClick += (_, _) =>
+        {
+            if (_lstUsers.SelectedItem is UserItem) DialogResult = DialogResult.OK;
+        };
+    }
+
+    protected override void OnFormClosing(FormClosingEventArgs e)
+    {
+        if (DialogResult == DialogResult.OK && _lstUsers.SelectedItem is UserItem item)
+            SelectedUserId = item.UserId;
+        base.OnFormClosing(e);
+    }
+
+    private sealed class UserItem(uint userId, string nickname)
+    {
+        public uint UserId { get; } = userId;
+        public override string ToString() => nickname;
+    }
+}