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/VoiceCat.App.csproj

@@ -18,6 +18,9 @@
          WinForms' own analyzer (WFO0003) flags manifest-based DPI settings as superseded by
          this property in modern .NET. -->
     <ApplicationHighDpiMode>PerMonitorV2</ApplicationHighDpiMode>
+    <!-- WASAPI process-loopback COM interop in Audio/ProcessLoopbackCapture.cs uses unsafe
+         pointer arithmetic for PROPVARIANT/AUDIOCLIENT_ACTIVATION_PARAMS marshaling. -->
+    <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
   </PropertyGroup>
 
   <!-- System.Security.Cryptography.ProtectedData (PasswordProtector.cs's DPAPI wrapper) ships