feat(windows): wire screen-audio sharing into the WinForms client

The core already supported SCREEN_AUDIO capture on Windows (post-M3 WASAPI
loopback via VOICECAT_HAS_LOOPBACK) and the C# Interop layer was complete
(VcStreamKind.ScreenAudio, StartStream/StopStream/SetRemoteStream). Only the
UI was missing -- no core, proto, or C ABI changes needed.

Adds a 'Share Screen Audio' toggle to the voice panel, independent of mic
voice (can share without joining voice). Disconnect/teardown now stops the
screen stream cleanly. New smoke test exercises the full StartStream ->
StreamStarted -> StopStream -> StreamStopped path through P/Invoke.

PROGRESS.md

@@ -10,6 +10,44 @@ up instantly. Newest status at the top.
 
 ## ▶ Where we left off / next action
 
+- **Done:** **Screen-audio sharing wired into the Windows WinForms client** (2026-06-17).
+  The core already fully supported `SCREEN_AUDIO` capture on Windows (post-M3 WASAPI
+  loopback via `VOICECAT_HAS_LOOPBACK`, always on for the `windows-client` preset —
+  `core/CMakeLists.txt:85`; loopback start/stop at `client.cpp:1093`/`audio_engine.cpp:529`;
+  VAD/PTT/self-mute/server-mute correctly bypassed for non-MIC kinds at `client.cpp:862-879`)
+  and the C# Interop layer was already complete (`VcStreamKind.ScreenAudio`,
+  `StartStream`/`StopStream`/`SetRemoteStream`/`ListUserStreams` all generic). The gap was
+  purely UI wiring. **No core, proto, or C ABI changes were needed** — confirming the
+  "the core should support it already" assessment.
+  - `MainForm.Designer.cs` — new `btnScreenShareToggle` button in the voice panel top row
+    (`flpVoiceTop`), right after `btnMicToggle`, with full `AccessibleName`/
+    `AccessibleDescription` per the existing accessibility convention.
+  - `MainForm.cs` — new `_screenStreamId` field; `BtnScreenShareToggle_Click` handler
+    mirroring `BtnMicToggle_Click` but with no device picker / VAD / PTT / mode / mute
+    (screen audio bypasses all of those in the core). Independent of mic — can share
+    without joining voice and vice versa. `HandleDisconnected` now resets
+    `_screenStreamId` and disables the screen toggle. `OnFormClosed` now explicitly stops
+    both mic and screen streams before `Disconnect()` (clean `StreamStop` messages go out
+    before the control channel closes). `HandleStreamStarted` already labeled
+    `ScreenAudio => "screen audio"`; `PerUserTuningDialog.ApplySettings` already iterates
+    all of a peer's streams — both unchanged, peers can independently volume-tune a
+    screen-audio stream vs that user's mic.
+  - `VoiceCatClientSmokeTests.cs` — new `ScreenAudioStream_Starts_And_Stops` test:
+    connects + TOFU + guest auth, `StartStream(ScreenAudio)`, asserts
+    `VC_EVENT_STREAM_STARTED` arrives with matching `StreamId`, `StopStream`, asserts
+    `VC_EVENT_STREAM_STOPPED`. Passes headless (the `StreamAnnounce` succeeds regardless
+    of whether the loopback device initializes on a CI box).
+  - `dotnet build` — 0 warnings/errors. `dotnet test` — **4/4 tests green**
+    (3 existing + 1 new). Event trace confirms the full
+    `StreamStarted → UserUpdated → StreamStopped` path through P/Invoke against a live
+    `voicecat-server.exe`.
+  - **Not yet confirmed audible by ear** — pending manual two-instance live test (one
+    shares screen audio while something plays on the default render endpoint, the other
+    hears it). This is the observable-behavior exit criterion per `AGENTS.md`.
+  - **Documented caveat** (`docs/voice.md §9`, unchanged): whole-device WASAPI loopback
+    inherently re-captures this app's own incoming voice mix (self-echo loop) — accepted
+    characteristic, not a bug. Process-specific loopback (Windows 10 2004+
+    `AUDIOCLIENT_ACTIVATION_PARAMS`) is a future enhancement; miniaudio doesn't expose it.
 - **In progress:** **M5 — moderation & admin** (2026-06-17). Server-side and C ABI are
   implemented and tested: permissions, kick/ban/move/server-mute, channel CRUD, in-app account
   management. Four new tests pass: `test_m5_permissions`, `test_m5_kick_ban_move_mute`,