Route startup storage paths through platform services

src/app.h

@@ -28,6 +28,7 @@
 
 namespace pp::platform {
 class PlatformServices;
+struct PlatformStoragePaths;
 }
 
 #if defined(__OBJC__) && defined(__IOS__)
@@ -205,6 +206,7 @@ public:
         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;
+    [[nodiscard]] pp::platform::PlatformStoragePaths prepare_storage_paths();
     void showKeyboard();
     void hideKeyboard();
     void initLog();