feat(windows-client): WinUI 3 (windows-reactor) UI — host list, settings, pairing, SwapChainPanel present

Replaces the winit + raw-HWND-D3D11 shell with a native WinUI 3 UI via windows-reactor (a
declarative React-like framework backed by WinUI). The earlier "Reactor can't host a
swapchain" read was wrong — PR #4499 (merged 2026-06-01) added a SwapChainPanel widget with
`set_swap_chain` over `CreateSwapChainForComposition`. Builds + clippy + fmt green on
x86_64-pc-windows-msvc.

- Cargo: drop winit/raw-window-handle; add windows-reactor + the `windows` crate, both pinned
  to the SAME windows-rs commit (b4129fcc) so the `IDXGISwapChain1` handed to `set_swap_chain`
  satisfies reactor's `windows_core::Interface`. Reactor's build.rs downloads the Windows App
  SDK NuGets + stages the bootstrap DLL/resources.pri — it requires `CARGO_WORKSPACE_DIR` set
  (now in the VM build env); /temp + /winmd gitignored.
- present.rs: composition swapchain (B8G8R8A8 FLIP_SEQUENTIAL premultiplied) bound to the
  SwapChainPanel; WARP fallback, runtime D3DCompile shaders, dynamic RGBA texture, Contain-fit
  letterbox; driven by reactor's per-frame `on_rendering`.
- app.rs: the WinUI 3 shell — host list (live mDNS + saved + manual), settings (resolution/
  refresh/mic combos+toggle), in-app SPAKE2 PIN pairing screen, and the stream page. Trust gate
  mirrors the GTK client (pinned → silent, pair=optional → TOFU, else PIN); a pinned-fp
  mismatch routes to re-pair. The session pump + decoded-frame handoff cross to the UI thread
  via a Mutex side-channel + thread-locals (the SwapChainPanel sample's pattern).
- gamepad: `ctl` sender now `Arc<Mutex<…>>` so GamepadService is `Sync` (shared across the UI
  and session-pump threads). main.rs: windowed = in-app UI; `--headless`/`--discover` keep the
  CLI paths.

Not yet wired: raw stream keyboard/mouse input (next commit — reactor exposes no raw key/
pointer events, so it needs Win32 low-level hooks or Microsoft.UI.Xaml bindings). On-glass
validation pending a display (the dev VM is headless/GPU-less).

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

crates/punktfunk-client-windows/src/gamepad.rs

@@ -51,7 +51,9 @@ pub struct GamepadService {
     pads: Arc<Mutex<Vec<PadInfo>>>,
     active: Arc<Mutex<Option<PadInfo>>>,
     pinned: Arc<Mutex<Option<u32>>>,
-    ctl: Sender<Ctl>,
+    // `Arc<Mutex<…>>` (not a bare `Sender`, which is `!Sync`) so the service is `Sync` — the
+    // WinUI app shares it across the UI thread and the session-pump thread (attach/detach).
+    ctl: Arc<Mutex<Sender<Ctl>>>,
 }
 
 impl GamepadService {
@@ -75,7 +77,7 @@ impl GamepadService {
             pads,
             active,
             pinned,
-            ctl,
+            ctl: Arc::new(Mutex::new(ctl)),
         }
     }
 
@@ -95,15 +97,15 @@ impl GamepadService {
 
     #[allow(dead_code)] // consumed by the settings GUI (follow-up)
     pub fn set_pinned(&self, id: Option<u32>) {
-        let _ = self.ctl.send(Ctl::Pin(id));
+        let _ = self.ctl.lock().unwrap().send(Ctl::Pin(id));
     }
 
     pub fn attach(&self, connector: Arc<NativeClient>) {
-        let _ = self.ctl.send(Ctl::Attach(connector));
+        let _ = self.ctl.lock().unwrap().send(Ctl::Attach(connector));
     }
 
     pub fn detach(&self) {
-        let _ = self.ctl.send(Ctl::Detach);
+        let _ = self.ctl.lock().unwrap().send(Ctl::Detach);
     }
 
     /// What "Automatic" resolves to right now — the virtual pad matching the physical one