Add separate dark mode logo with white text

Use two logo images — light (black text) and dark (white text) —
toggled via CSS instead of a brightness filter. Keeps the pink
icon intact in both modes.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: mgarbs

site/index.html

@@ -18,7 +18,8 @@
     <div class="header-inner">
       <div class="header-left">
         <a href="/" class="site-logo">
-          <img src="/hiero_logo_cropped.png" alt="Hiero" class="logo-img">
+          <img src="/hiero_logo_cropped.png" alt="Hiero" class="logo-img logo-light">
+          <img src="/hiero_logo_dark.png" alt="Hiero" class="logo-img logo-dark">
           <span class="logo-subtitle">Open Source Distributed Ledger Technology</span>
         </a>
         <nav class="header-nav">

site/public/hiero_logo_dark.png

@@ -141,8 +141,10 @@ a:hover { color: var(--link-h); }
 .header-left { display: flex; align-items: center; gap: 1.5rem; }
 .site-logo { display: flex; flex-direction: column; align-items: flex-start; text-decoration: none; gap: 2px; }
 .logo-img { height: 32px; width: auto; }
+.logo-dark { display: none; }
 .logo-subtitle { font-size: .75rem; font-weight: 500; color: var(--fg-muted); letter-spacing: .02em; }
-[data-theme="dark"] .logo-img { filter: brightness(1.8); }
+[data-theme="dark"] .logo-light { display: none; }
+[data-theme="dark"] .logo-dark { display: block; }
 
 .header-nav { display: flex; gap: 2px; }
 .nav-link {