Add Prolific guidance banner to login page

ls_proxy_hf.py

@@ -1228,6 +1228,12 @@ _LOGIN_BANNER = """
     Sign in with your email and password  · 
     <a href="/user/signup/" style="color:#e8c97a;text-decoration:underline;">Create account</a>
   </div>
+</div>
+<div style="background:#fff3cd;border:1px solid #ffc107;border-radius:8px;margin:16px 24px 0;
+            padding:12px 16px;font-family:sans-serif;font-size:13px;color:#856404;text-align:center;">
+  <b>Coming from Prolific?</b> Do not log in here — use the <b>Start</b> button in your Prolific study link instead.
+  Your login happens automatically when you click that link.
+  If you lost the link, return to <a href="https://app.prolific.com" style="color:#856404;">app.prolific.com</a> and open the study from there.
 </div>"""