fix: hide + freeze the local cursor while streaming

The host renders its own cursor from our raw deltas, so the local macOS cursor both
stays visible and drifts away from the remote one — and it can wander out of the window,
where a click focuses another app. While the stream has focus, do what Moonlight does:
warp the cursor mid-view, disconnect it from mouse movement
(CGAssociateMouseAndMouseCursorPosition(false) — GCMouse still delivers raw HID deltas),
and hide it. Released on app deactivation (Cmd+Tab is the escape hatch), view teardown,
and disconnect; re-captured when the stream regains focus. The HUD's Disconnect gains ⌘D
since a hidden, frozen cursor can't click it.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

clients/apple/Sources/PunktfunkClient/ContentView.swift

@@ -65,8 +65,11 @@ struct ContentView: View {
         VStack(alignment: .trailing, spacing: 4) {
             Text("\(conn.width)×\(conn.height)@\(conn.refreshHz)  \(model.fps) fps  \(model.mbps, specifier: "%.1f") Mb/s")
                 .font(.system(.caption, design: .monospaced))
-            Button("Disconnect") { model.disconnect() }
+            // ⌘D because the local cursor is hidden+frozen while streaming — the button
+            // can't be clicked. (Cmd+Tab away also frees the cursor.)
+            Button("Disconnect (⌘D)") { model.disconnect() }
                 .font(.caption)
+                .keyboardShortcut("d", modifiers: .command)
         }
         .padding(8)
         .background(.black.opacity(0.5), in: RoundedRectangle(cornerRadius: 6))