feat(windows): per-app screen-audio sharing (include/exclude)

Adds "only selected apps" and "all apps except selected" screen-audio
modes to the Windows client, alongside the existing entire-desktop path.

Per-app capture uses WASAPI process loopback (AUDCLNT_ACTIVATIONTYPE_
PROCESS_LOOPBACK) via ProcessLoopbackCapture, mixed by ProcessAudioMixer
and fed to the core through vc_stream_feed_pcm (external_feed=1 so the
core skips its own loopback device).

Init must pass AUDCLNT_STREAMFLAGS_LOOPBACK | EVENTCALLBACK |
AUTOCONVERTPCM; the LOOPBACK flag is what makes the virtual endpoint
deliver rendered audio (without it every buffer is flagged SILENT) and
AUTOCONVERTPCM resamples the app's native format to 48k s16.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

clients/windows/VoiceCat.App/Forms/AppAudioPickerDialog.cs

@@ -0,0 +1,166 @@
+using VoiceCat.App.Audio;
+
+namespace VoiceCat.App.Forms;
+
+/// <summary>
+/// Modal dialog for selecting which apps' audio to share.
+/// Returns the chosen <see cref="AppAudioScope"/> via <see cref="ChosenScope"/>,
+/// or <see cref="DialogResult.Cancel"/> if the user dismissed without sharing.
+/// </summary>
+public sealed class AppAudioPickerDialog : Form
+{
+    private readonly RadioButton  _rdoAll;
+    private readonly RadioButton  _rdoOnly;
+    private readonly RadioButton  _rdoExcept;
+    private readonly ListView     _appList;
+    private readonly Label        _lblApps;
+
+    // Snapshot taken when the dialog opens (refresh on open, not on every check change).
+    private IReadOnlyList<AudioAppInfo> _apps = [];
+
+    public AppAudioScope? ChosenScope { get; private set; }
+
+    public AppAudioPickerDialog()
+    {
+        // ── Radio buttons ───────────────────────────────────────────────────
+        _rdoAll = new RadioButton
+        {
+            Text      = "&Entire desktop",
+            Checked   = true,
+            Location  = new Point(12, 12),
+            Size      = new Size(360, 20),
+            TabIndex  = 0,
+        };
+        _rdoOnly = new RadioButton
+        {
+            Text     = "&Only selected apps",
+            Location = new Point(12, 36),
+            Size     = new Size(360, 20),
+            TabIndex = 1,
+        };
+        _rdoExcept = new RadioButton
+        {
+            Text     = "All apps e&xcept selected",
+            Location = new Point(12, 60),
+            Size     = new Size(360, 20),
+            TabIndex = 2,
+        };
+
+        _rdoAll.CheckedChanged    += OnModeChanged;
+        _rdoOnly.CheckedChanged   += OnModeChanged;
+        _rdoExcept.CheckedChanged += OnModeChanged;
+
+        // ── App list ────────────────────────────────────────────────────────
+        _lblApps = new Label
+        {
+            Text     = "Apps with active audio sessions:",
+            AutoSize = true,
+            Location = new Point(12, 90),
+            Visible  = false,
+            TabIndex = 3,
+        };
+
+        _appList = new ListView
+        {
+            Location      = new Point(12, 112),
+            Size          = new Size(360, 180),
+            CheckBoxes    = true,
+            View          = View.List,
+            Visible       = false,
+            TabIndex      = 4,
+            FullRowSelect = true,
+        };
+
+        // ── Buttons ─────────────────────────────────────────────────────────
+        var btnShare = new Button
+        {
+            Text         = "&Share",
+            DialogResult = DialogResult.OK,
+            Location     = new Point(216, 308),
+            Size         = new Size(75, 27),
+            TabIndex     = 5,
+        };
+        var btnCancel = new Button
+        {
+            Text         = "&Cancel",
+            DialogResult = DialogResult.Cancel,
+            Location     = new Point(297, 308),
+            Size         = new Size(75, 27),
+            TabIndex     = 6,
+        };
+        btnShare.Click += OnShareClick;
+
+        AcceptButton  = btnShare;
+        CancelButton  = btnCancel;
+        AutoScaleMode = AutoScaleMode.Font;
+        ClientSize    = new Size(384, 348);
+        Controls.AddRange([_rdoAll, _rdoOnly, _rdoExcept, _lblApps, _appList, btnShare, btnCancel]);
+        FormBorderStyle = FormBorderStyle.FixedDialog;
+        MaximizeBox     = false;
+        MinimizeBox     = false;
+        StartPosition   = FormStartPosition.CenterParent;
+        Text            = "Share App Audio";
+    }
+
+    protected override void OnLoad(EventArgs e)
+    {
+        base.OnLoad(e);
+        RefreshAppList();
+    }
+
+    private void RefreshAppList()
+    {
+        _apps = AudioSessionEnumerator.GetAudioApps();
+        _appList.Items.Clear();
+        foreach (var app in _apps)
+            _appList.Items.Add(new ListViewItem($"{app.DisplayName}  (PID {app.Pid})") { Tag = app.Pid });
+    }
+
+    private void OnModeChanged(object? sender, EventArgs e)
+    {
+        bool showList = _rdoOnly.Checked || _rdoExcept.Checked;
+        _lblApps.Visible = showList;
+        _appList.Visible = showList;
+
+        if (showList && _appList.Items.Count == 0)
+            RefreshAppList();
+    }
+
+    private void OnShareClick(object? sender, EventArgs e)
+    {
+        if (_rdoAll.Checked)
+        {
+            ChosenScope = new EntireDesktop();
+            return;
+        }
+
+        var checkedPids   = new List<int>();
+        var checkedNames  = new List<string>();
+        foreach (ListViewItem item in _appList.CheckedItems)
+        {
+            if (item.Tag is int pid)
+            {
+                checkedPids.Add(pid);
+                checkedNames.Add(item.Text);
+            }
+        }
+
+        if (checkedPids.Count == 0)
+        {
+            MessageBox.Show(
+                "Select at least one app, or choose 'Entire desktop'.",
+                "No apps selected",
+                MessageBoxButtons.OK,
+                MessageBoxIcon.Warning);
+            DialogResult = DialogResult.None; // prevent close
+            return;
+        }
+
+        ChosenScope = _rdoOnly.Checked
+            ? new OnlyApps(checkedPids, checkedNames)
+            : new AllExceptApps(checkedPids, checkedNames);
+    }
+
+    /// <summary>All apps that were visible in the list when the user clicked Share.</summary>
+    public IReadOnlyList<AudioAppInfo> VisibleApps => _apps;
+}

clients/windows/VoiceCat.App/Forms/MainForm.cs

@@ -1,3 +1,4 @@
+using VoiceCat.App.Audio;
 using VoiceCat.Interop;
 
 namespace VoiceCat.App.Forms;
@@ -22,6 +23,7 @@ public partial class MainForm : Form
     // Voice state
     private uint _micStreamId; // 0 = not started
     private uint _screenStreamId; // 0 = not sharing screen audio
+    private ProcessAudioMixer? _screenMixer; // non-null only in per-app capture mode
     private Keys _pttKey = Keys.F8;
     private bool _serverMuted;
     private bool _serverDeafened;
@@ -442,6 +444,7 @@ public partial class MainForm : Form
         _currentChannelId = 0;
         _micStreamId = 0;
         _screenStreamId = 0;
+        _screenMixer?.Stop(); _screenMixer?.Dispose(); _screenMixer = null;
         txtCompose.Enabled = false;
         btnSend.Enabled = false;
         tsbJoinVoice.Enabled = false;
@@ -622,29 +625,73 @@ public partial class MainForm : Form
     {
         if (_screenStreamId == 0)
         {
-            var (result, streamId) = _client.StartStream(VcStreamKind.ScreenAudio, "Desktop audio");
-            if (result == VcResult.Ok)
-            {
-                _screenStreamId = streamId;
-                tsbScreenShare.Text = "Stop Screen Audio";
-                _miScreenShare.Text = "Stop Screen &Audio";
-                AddActivity("Started sharing screen audio");
-            }
-            else
-            {
-                AddActivity($"Failed to start screen audio: {result}");
-            }
+            StartScreenAudio();
         }
         else
         {
-            _client.StopStream(_screenStreamId);
-            _screenStreamId = 0;
-            tsbScreenShare.Text = "Share Screen Audio";
-            _miScreenShare.Text = "Share Screen &Audio";
-            AddActivity("Stopped sharing screen audio");
+            StopScreenAudio();
         }
     }
 
+    private void StartScreenAudio()
+    {
+        using var picker = new AppAudioPickerDialog();
+        if (picker.ShowDialog(this) != DialogResult.OK || picker.ChosenScope == null) return;
+
+        var scope = picker.ChosenScope;
+
+        if (scope is EntireDesktop)
+        {
+            // Existing whole-device WASAPI loopback path — core handles it.
+            var (result, streamId) = _client.StartStream(VcStreamKind.ScreenAudio, "Desktop audio");
+            if (result != VcResult.Ok)
+            {
+                AddActivity($"Failed to start screen audio: {result}");
+                return;
+            }
+            _screenStreamId = streamId;
+            AddActivity("Sharing screen audio: entire desktop");
+        }
+        else
+        {
+            // Per-app path: suppress core loopback, C# mixer feeds PCM.
+            var (result, streamId) = _client.StartStreamExternalFeed(VcStreamKind.ScreenAudio, "App audio");
+            if (result != VcResult.Ok)
+            {
+                AddActivity($"Failed to start screen audio: {result}");
+                return;
+            }
+            _screenStreamId = streamId;
+
+            _screenMixer = new ProcessAudioMixer();
+            _screenMixer.Start(scope, _client, streamId, picker.VisibleApps);
+
+            string desc = scope switch
+            {
+                OnlyApps    o => $"only {o.Pids.Count} app(s)",
+                AllExceptApps a => $"all except {a.Pids.Count} app(s)",
+                _ => "apps",
+            };
+            AddActivity($"Sharing screen audio: {desc}");
+        }
+
+        tsbScreenShare.Text = "Stop Screen Audio";
+        _miScreenShare.Text = "Stop Screen &Audio";
+    }
+
+    private void StopScreenAudio()
+    {
+        _screenMixer?.Stop();
+        _screenMixer?.Dispose();
+        _screenMixer = null;
+
+        _client.StopStream(_screenStreamId);
+        _screenStreamId = 0;
+        tsbScreenShare.Text = "Share Screen Audio";
+        _miScreenShare.Text = "Share Screen &Audio";
+        AddActivity("Stopped sharing screen audio");
+    }
+
     private void TrkOutputVolume_Scroll(object? sender, EventArgs e) =>
         _client.SetOutputVolume(trkOutputVolume.Value / 100f);
 
@@ -1019,7 +1066,7 @@ public partial class MainForm : Form
         _client.EventReceived -= OnEvent;
         foreach (var win in _pmWindows.Values.ToList()) win.Close();
         _pmWindows.Clear();
-        if (_screenStreamId != 0) _client.StopStream(_screenStreamId);
+        if (_screenStreamId != 0) { _screenMixer?.Stop(); _screenMixer?.Dispose(); _screenMixer = null; _client.StopStream(_screenStreamId); }
         if (_micStreamId != 0) _client.StopStream(_micStreamId);
         _client.Disconnect();
         _client.Dispose();