fix: add basePath to SessionProvider for client-side auth routing

The client-side signIn()/signOut() functions from next-auth/react
default to /api/auth. Since we moved NextAuth to /auth/api to avoid
DO ingress prefix stripping, SessionProvider needs basePath="/auth/api"
to match the server config.

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

Author: ComBba

web/src/components/providers/session-provider.tsx

@@ -7,5 +7,5 @@ export function AuthSessionProvider({
 }: {
   children: React.ReactNode;
 }) {
-  return <SessionProvider refetchInterval={300}>{children}</SessionProvider>;
+  return <SessionProvider basePath="/auth/api" refetchInterval={300}>{children}</SessionProvider>;
 }