[NIVEL 3] Issue #56 - Related Posts Background Gris con Borde Lateral Animado

Implementación completa de estilos relacionados posts según diseño RDash.

**Cambios en related-posts.css:**
- Background: #f7fafc (normal) → #ffffff (hover)
- Border: 1px solid #e2e8f0
- Border-left animado con ::before pseudo-element
- Gradiente border-left: linear-gradient(180deg, #1e3a5f → #1a73e8)
- Opacity border: 0 (normal) → 1 (hover)
- Hover transform: translateY(-8px) (antes -4px)
- Shadow hover: 0 12px 32px rgba(26, 115, 232, 0.15)
- Transición: cubic-bezier(0.4, 0, 0.2, 1) mantiene
- Agregado soporte prefers-reduced-motion para ::before

**Características:**
✅ Background gris #f7fafc con transición a blanco
✅ Borde lateral 4px gradiente animado (opacity)
✅ Hover elevación -8px con shadow profesional
✅ Gradiente azul (#1e3a5f → #1a73e8) vertical
✅ Transición cubic-bezier suave
✅ Accessibility: reduced motion support

Closes #56

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

wp-content/themes/apus-theme/assets/css/related-posts.css

@@ -52,7 +52,10 @@
     display: flex;
     flex-direction: column;
     height: 100%;
-    background: #ffffff;
+    background: #f7fafc;
+    border: 1px solid #e2e8f0;
+    border-left: 4px solid transparent;
+    border-left-color: transparent;
     border-radius: 12px;
     overflow: hidden;
     box-shadow: 0 2px 8px rgba(0, 0, 0, 0.08);
@@ -60,9 +63,27 @@
     position: relative;
 }
 
+.related-post-card::before {
+    content: '';
+    position: absolute;
+    left: 0;
+    top: 0;
+    bottom: 0;
+    width: 4px;
+    background: linear-gradient(180deg, #1e3a5f 0%, #1a73e8 100%);
+    opacity: 0;
+    transition: opacity 0.3s cubic-bezier(0.4, 0, 0.2, 1);
+    z-index: 1;
+}
+
 .related-post-card:hover {
-    transform: translateY(-4px);
-    box-shadow: 0 8px 24px rgba(0, 0, 0, 0.12);
+    transform: translateY(-8px);
+    box-shadow: 0 12px 32px rgba(26, 115, 232, 0.15);
+    background: #ffffff;
+}
+
+.related-post-card:hover::before {
+    opacity: 1;
 }
 
 .related-post-link {
@@ -444,6 +465,7 @@
 /* Reduce motion for users who prefer it */
 @media (prefers-reduced-motion: reduce) {
     .related-post-card,
+    .related-post-card::before,
     .related-post-thumbnail img,
     .related-post-category,
     .related-post-title {