style: stronger bg contrast on landing page for Windows

Darken --bg (#e4eaf2 → #d9e1ec) and --border (#dce3ed → #c9d3e0)
for clearly visible card separation on Windows displays.

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

Author: jmacot

index.html

@@ -23,11 +23,11 @@
   <link href="https://fonts.googleapis.com/css2?family=DM+Serif+Display:ital@0;1&family=DM+Mono:wght@400;500&family=DM+Sans:wght@300;400;500&display=swap" rel="stylesheet" />
   <style>
     :root {
-      --bg: #e4eaf2;
+      --bg: #d9e1ec;
       --surface: #ffffff;
       --ink: #0f172a;
       --ink-muted: #64748b;
-      --border: #dce3ed;
+      --border: #c9d3e0;
 
       /* Sombras modernas */
       --shadow-sm: 0 1px 3px 0 rgb(0 0 0 / 0.06), 0 1px 2px -1px rgb(0 0 0 / 0.06);