Extract file menu binding and Win32 splash helper

src/platform_windows/windows_splash.h

@@ -0,0 +1,30 @@
+#pragma once
+
+#include <windows.h>
+#include <atomic>
+#include <thread>
+
+namespace pp::platform::windows {
+
+class SplashScreen final
+{
+public:
+    explicit SplashScreen(HINSTANCE hInst);
+    ~SplashScreen();
+
+    SplashScreen(const SplashScreen&) = delete;
+    SplashScreen& operator=(const SplashScreen&) = delete;
+    SplashScreen(SplashScreen&&) = delete;
+    SplashScreen& operator=(SplashScreen&&) = delete;
+
+    void dismiss();
+
+private:
+    void thread_main();
+
+    HINSTANCE hInst_{};
+    std::atomic<HWND> dialog_{};
+    std::jthread thread_;
+};
+
+}