Modularize CSS into page-specific files

Split style.css into home.css (hero, features, newsletter, splash) and
products.css (product grid/cards). Each page loads only what it needs
via $extraHead. style.css now contains only truly shared styles.

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

product.php

@@ -3,6 +3,7 @@
  * Tom's Java Jive - Product Detail Page
  */
 
+$extraHead = '<link rel="stylesheet" href="/assets/css/products.css?v=' . filemtime(__DIR__ . '/assets/css/products.css') . '">';
 require_once __DIR__ . '/includes/functions.php';
 
 $productId = $_GET['id'] ?? '';