Replace placeholder with a proper multi-section landing page

- public/index.html: semantic HTML5 with header/nav, hero, features grid,
  stack description list, contact, footer
- public/styles.css: responsive layout, custom properties, dark-mode support
- public/favicon.svg

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

public/favicon.svg

@@ -0,0 +1,10 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 32 32">
+  <defs>
+    <linearGradient id="g" x1="0" y1="0" x2="1" y2="1">
+      <stop offset="0" stop-color="#2b5bd7"/>
+      <stop offset="1" stop-color="#9bb5ff"/>
+    </linearGradient>
+  </defs>
+  <rect width="32" height="32" rx="7" fill="url(#g)"/>
+  <path d="M9 11h14M16 11v12" stroke="#fff" stroke-width="2.5" stroke-linecap="round"/>
+</svg>