Fix register endpoint to accept camelCase JSON fields

Frontend sends inviteCode but backend expected invite_code,
causing deserialization error on registration.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend_rust/src/api/auth.rs

@@ -9,6 +9,7 @@ use crate::error::ApiError;
 use crate::middleware::auth::create_token;
 
 #[derive(Debug, Deserialize)]
+#[serde(rename_all = "camelCase")]
 pub struct RegisterRequest {
     pub username: String,
     pub password: String,