Wall ownership ladder: positions 2 and 3 become the member's own at 2 / 5 qualifying buyers

- accounts.wall_offers (JSON, up to 2 offers: label, https link, banner) set from
  Profile > Your wall; upload supported; locks show the buyer threshold.
- /api/wall assembles: position 1 = own line banner; positions 2-3 = own offer
  when unlocked and set, else upline banners (only uplines with a live banner),
  else house ads. Response carries unlocked + buyerCount; wall labels show
  "this wall" / "their line" / "InstantAdPay".
- Chatbot canned answer + facts, follow-up email 7 mention the ladder.

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

public/join.html

@@ -6,7 +6,7 @@
 <title>You're invited | InstantAdPay</title>
 <link rel="stylesheet" href="https://fonts.googleapis.com/css2?family=Sora:wght@600;700;800&display=swap">
 <link rel="icon" type="image/png" href="/logo-icon.png">
-<link rel="stylesheet" href="/assets/site.css?v=20260909c">
+<link rel="stylesheet" href="/assets/site.css?v=20260909d">
 <style>
   .jn-nav{display:flex;justify-content:space-between;align-items:center;gap:12px;padding:16px 0}
   .jn-hero{padding:34px 0 10px;text-align:left}