Sponsor attribution: last touch (30-day cookie), locked at account creation; 'Joining under' line at the code step; chatbot answer

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

public/join.html

@@ -88,6 +88,7 @@
       <p class="muted small" style="margin:0 0 12px">Type your email and we send a 6-digit code. No password, no wallet needed today.</p>
       <p><input id="jnEmail" type="email" placeholder="Your email" autocomplete="email"></p>
       <p id="jnCodeRow" hidden><input id="jnCode" inputmode="numeric" placeholder="6-digit code from your inbox"></p>
+      <p id="jnUnder" class="small muted" hidden style="margin:-4px 0 12px"></p>
       <label class="small muted" style="display:flex;gap:8px;align-items:flex-start;margin:0 0 12px;cursor:pointer">
         <input type="checkbox" id="jnNews" checked style="margin-top:3px;width:auto">
         <span>Send me the getting-started emails (a few short ones over the first week). Unsubscribe any time.</span>
@@ -128,6 +129,6 @@
   </div>
 </div>
 <script src="/assets/common.js?v=20260910c"></script>
-<script src="/assets/join.js?v=20260910a"></script>
+<script src="/assets/join.js?v=20260910b"></script>
 </body>
 </html>