feat: add web support for powersync attachements helper

[bennajah]

add support for using the PowerSync Attachments Helper in Flutter Web applications.

[simolus3]

Thanks for the request! Where would you expect files to be stored in this case, OPFS? Given that the web doesn't really have an obvious file system implementation, I think it'll be tricky for us to support web out of the box.

We could definitely look into APIs that make it easier to replace dart:io with something custom, but it would still require a fair bit of customization.

[bennajah]

Yes, OPFS would be the ideal solution for persistent file storage on the web, with idb_shim serving as a reliable fallback for browsers that do not support OPFS.

[keeandev]

@simolus3 Any updates on this?

[simolus3]

We've recently released a new attachments helper system that at least compiles to the web, but doesn't have a builtin storage implementation for OPFS or IndexedDb yet. It would let you write your own LocalStorage implementation though, which you could use for web support.

[simolus3]

I've experimented with attachments on the web in this branch. It may take us a while to release that, but you could also copy the OpfsLocalStorage implementation into your project to use it - it just implements a public interface.