Sponsor to downline messaging: lineage view, daily broadcast (email+inbox), login modal

- Downline resolver (3 levels); email visible for directs only, username+ID deeper
- Broadcast composer (rich editor) to directs or full downline, 1/day cap, sends email + on-site inbox
- Upline messages inbox in My line; unmissable sign-in modal for unread, ack marks read
- messages.js dual-mode store; sponsor_messages table; sanitizeRich reused

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

public/view.html

@@ -4,7 +4,7 @@
 <meta charset="utf-8">
 <meta name="viewport" content="width=device-width,initial-scale=1">
 <title>Viewing ad — InstantAdPay</title>
-<link rel="stylesheet" href="/assets/site.css?v=20260905y">
+<link rel="stylesheet" href="/assets/site.css?v=20260905z">
 <style>
   html,body{height:100%;margin:0;overflow:hidden}
   .vw{display:flex;flex-direction:column;height:100vh;height:100dvh;background:var(--bg,#04110c);color:var(--ink,#e8fff7)}
@@ -43,6 +43,6 @@
   </div>
   <iframe class="vframe" id="vFrame" sandbox="allow-scripts allow-same-origin allow-forms allow-popups" referrerpolicy="no-referrer" title="Advertiser site"></iframe>
 </div>
-<script src="/assets/view.js?v=20260905y"></script>
+<script src="/assets/view.js?v=20260905z"></script>
 </body>
 </html>