Replace UNREFERENCED_PARAMETER with NEBULA_UNUSED

Introduce NEBULA_UNUSED macro in platform types and replace usages of UNREFERENCED_PARAMETER across multiple source files to standardize unused-parameter handling. Update various platform stubs (Linux window/host), CEF handlers, and Windows startup code to use NEBULA_UNUSED. Also adjust NebulaController::OnWindowCloseRequested to stop force-destroying the top-level window and rely on MaybeFinishShutdown once browsers report OnBeforeClose.

app/main_bigpicture.cpp

@@ -12,8 +12,8 @@ int APIENTRY wWinMain(HINSTANCE instance,
                       HINSTANCE previous_instance,
                       LPWSTR command_line,
                       int show_command) {
-    UNREFERENCED_PARAMETER(previous_instance);
-    UNREFERENCED_PARAMETER(command_line);
+    NEBULA_UNUSED(previous_instance);
+    NEBULA_UNUSED(command_line);
 
     const nebula::platform::AppStartup startup{instance, show_command};
     return nebula::app::RunNebula(startup, {nebula::app::AppMode::BigPicture});