Ship logo (header + favicon) + bolder downline level labels

- Logo lockup (mint lightning/arrow icon + InstantAdPay wordmark) replaces the
  text wordmark in the back-office sidebar and public nav; icon added as the
  favicon on every page. Assets: /logo.png, /logo-icon.png.
- Downline level captions (LEVEL 1 / LEVEL 2 …) are now a bold mint pill instead
  of faint grey — far more readable.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

public/assets/common.js

@@ -27,7 +27,7 @@ window.IAP = (function () {
     const c = await getConfig();
     const nav = document.createElement('nav');
     nav.innerHTML = '<div class="wrap">'
-      + '<a class="logo" href="/">Instant<b>AdPay</b></a>'
+      + '<a class="logo" href="/"><img src="/logo.png" alt="InstantAdPay" style="height:30px;display:block"></a>'
       + '<span class="links">'
       + '<a href="/" data-p="home">How it works</a>'
       + '<a href="/#ladder" data-p="pricing">Ad packages</a>'