Could I ask why you need web app? There are plenty of great desktop applications that will work well, sync with your phone, and can be accessed when you don't have an internet connection.

Everybody knows Obsidian, but you might think that's overkill. It's also closed-source unfortunately. I use Marktext, which is open-source and more lightweight than Obsidian. I don't think it's developed anymore, but the core functionality works well enough.

You might also want to use VSCodium, as that also supports Markdown files, or perhaps some other open-source editor, like Kate!

As for Android apps, I can recommend Markor as it is open-source and supports plain Markdown files w/ folder structure and all. Obsidian is also available on Android if you want that.

For syncing, I like Syncthing since it's peer-to-peer (does not require a server, so devices just need to be on the same network to sync). You have a remote device, so server-based syncing will probably work better for you. In that case, I can recommend Nextcloud for that, it works well and has clients for all the different platforms. You can self-host it or use one of the many public instances! There are other options too, like OpenCloud and Seafile, but I haven't used those myself. You could also use another cloud storage provider, there are plenty of those that offer sync features.