Route prepared export policy through platform services

src/app.h

@@ -184,6 +184,7 @@ public:
     void pick_file_save(const std::string& type, const std::string& default_name,
         std::function<void(std::string path)> writer, std::function<void(const std::string& path, bool saved)> callback);
     void pick_file_save(std::vector<std::string> types, std::function<void(std::string path)> callback);
+    [[nodiscard]] bool uses_prepared_file_writes() const;
     void pick_dir(std::function<void(std::string path)> callback);
     void display_file(std::string path);
     void share_file(std::string path);