Route prepared file saves through platform services

src/app.h

@@ -189,6 +189,10 @@ public:
     void pick_dir(std::function<void(std::string path)> callback);
     void display_file(std::string path);
     void share_file(std::string path);
+    void save_prepared_file(
+        std::string path,
+        std::string suggested_name,
+        std::function<void(const std::string& path, bool saved)> callback);
     void set_platform_services(pp::platform::PlatformServices* services) noexcept;
     [[nodiscard]] pp::platform::PlatformServices* platform_services() const noexcept;
     void showKeyboard();