Remote /prove endpoint + desktop Linux WebKit fix

For users on phones or low-end laptops, generating a Groth16 proof in 1.5s is still 1.5s of dead UI. Added a /prove endpoint on vanta-explorer that takes a witness, runs the prover server-side, and returns the 128-byte proof. Trust trade-off documented in the desktop app — server sees the witness, but only the user sees the resulting note keys. Same commit fixes the Linux WebKit build for…