Update synapse-react to support multi sp upload flow

[hugomrdias]

either:

• use core only, may be useful to validate the primitives we have there but more work
• use sdk, easy path sdk already does all the work

Demo app should also be updated to use it.

[BigLep]

2026-03-03 FOC engineering group sync agreed this should happen, but it's near the bottom of the M4.1 stack given we also have other examples with https://fs-upload-dapp.netlify.app/ and filecoin-pin.