Route prepared file saves through platform services

src/platform_legacy/legacy_platform_services.cpp

@@ -202,6 +202,29 @@ public:
         });
 #else
         (void)value;
+#endif
+    }
+
+    void save_prepared_file(
+        std::string_view path,
+        std::string_view suggested_name,
+        pp::platform::PreparedFileCallback callback) override
+    {
+        const std::string value(path);
+        const std::string name(suggested_name);
+#ifdef __IOS__
+        (void)name;
+        dispatch_async(dispatch_get_main_queue(), ^{
+            [App::I->ios_view pick_file_save:value];
+        });
+        callback(value, true);
+#elif __WEB__
+        webgl_pick_file_save(value, name, [callback = std::move(callback), value](bool success) {
+            callback(value, success);
+        });
+#else
+        (void)name;
+        callback(value, false);
 #endif
     }
 };