Route prepared file saves through platform services

src/platform_windows/windows_platform_services.cpp

@@ -201,6 +201,15 @@ public:
         const std::string path = open_directory();
         invoke_selected_path(path, callback);
     }
+
+    void save_prepared_file(
+        std::string_view path,
+        std::string_view suggested_name,
+        pp::platform::PreparedFileCallback callback) override
+    {
+        (void)suggested_name;
+        callback(std::string(path), false);
+    }
 };
 
 }