Fix 401 redirect loop on login page

The 401 handler was redirecting to /login unconditionally, causing an
infinite reload loop when useTheme fired unauthenticated API calls on
the login page itself. Now only redirects if not already on /login.

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

frontend/src/api/client.ts

@@ -58,8 +58,9 @@ async function request<T>(
   // Global 401 handler — expired or invalid token
   if (response.status === 401) {
     localStorage.removeItem("token");
-    window.location.href = "/login";
-    // Throw so callers don't try to process the response
+    if (window.location.pathname !== "/login") {
+      window.location.href = "/login";
+    }
     throw new ApiError(401, "Session expired");
   }