fix(auth): suppress misleading 502 warning during session validation

A 502 Bad Gateway is a server/gateway error, not a network error.
Logging it as a 'Session validation network error' is noisy and
misleading during startup when nginx is temporarily unreachable.

Silently skip the console.warn for 5xx errors in handleValidationError
while keeping the warning for actual network errors.

frontend/src/providers/AuthProvider.tsx

@@ -56,7 +56,7 @@ import React, {
 } from "react";
 import { useNavigate } from "react-router-dom";
 import * as authApi from "../api/auth";
-import { setUnauthorizedHandler, resetLogoutState, clearSessionCorrelationId } from "../api/client";
+import { ApiError, setUnauthorizedHandler, resetLogoutState, clearSessionCorrelationId } from "../api/client";
 import { setAuthErrorHandler, resetLogoutState as resetFetchErrorLogoutState } from "../utils/fetchError";
 import { STORAGE_KEY_AUTHENTICATED } from "../utils/constants";
 import { SessionValidationLoading } from "../components/SessionValidationLoading";
@@ -133,6 +133,11 @@ export function AuthProvider({
 
   const handleValidationError = useCallback(
     (error: Error): void => {
+      // Suppress noisy warning for 5xx gateway errors (e.g. 502 Bad Gateway)
+      // during startup — these are server-side issues, not network issues.
+      if (error instanceof ApiError && error.status >= 500) {
+        return;
+      }
       // Network error — log but don't logout.
       console.warn("Session validation network error:", error);
     },