Fix EC-SRP5 server: use lift_x not redp1 for verification

Server-side shared secret used redp1(x_gamma) which is the hash-to-curve
blinding function, but verification needs lift_x(x_gamma) — the raw
validator public key point. Also fixed prime_mod_sqrt for p ≡ 5 (mod 8)
using Atkin's algorithm instead of Tonelli-Shanks.

Removed unused password parameter from server_authenticate.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/server.rs

@@ -211,7 +211,6 @@ async fn handle_client(
         crate::ecsrp5::server_authenticate(
             &mut stream,
             auth_user.as_deref().unwrap_or("admin"),
-            auth_pass.as_deref().unwrap_or(""),
             creds,
         )
         .await?;