feat(theme): add [roi_post_grid] shortcode for static pages

- Create PostGridShortcodeRegistrar for WordPress shortcode registration
- Implement RenderPostGridUseCase following Clean Architecture
- Add PostGridQueryBuilder for custom WP_Query construction
- Add PostGridShortcodeRenderer for HTML/CSS generation
- Register shortcode in DIContainer with proper DI
- Add shortcode usage guide in post-grid admin panel
- Fix sidebar layout: add hide_for_logged_in check to wrapper visibility

Shortcode attributes: category, tag, author, posts_per_page, columns,
show_pagination, show_thumbnail, show_excerpt, show_meta, etc.

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

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

Shared/Domain/Contracts/PostGridShortcodeRendererInterface.php

@@ -0,0 +1,50 @@
+<?php
+declare(strict_types=1);
+
+namespace ROITheme\Shared\Domain\Contracts;
+
+/**
+ * Interface PostGridShortcodeRendererInterface
+ *
+ * Contrato para renderizado HTML del shortcode post-grid.
+ * Define el comportamiento esperado para generar el HTML del grid
+ * sin depender de implementaciones especificas.
+ *
+ * Responsabilidades:
+ * - Generar HTML del grid de posts
+ * - Generar CSS inline usando CSSGeneratorInterface
+ * - Aplicar clases responsive de Bootstrap
+ *
+ * NO responsable de:
+ * - Construir queries
+ * - Obtener settings de BD
+ * - Sanitizar atributos del shortcode
+ *
+ * @package ROITheme\Shared\Domain\Contracts
+ */
+interface PostGridShortcodeRendererInterface
+{
+    /**
+     * Renderiza el grid de posts como HTML.
+     *
+     * Ejemplo:
+     * ```php
+     * $html = $renderer->render($query, $settings, [
+     *     'columns' => 3,
+     *     'show_thumbnail' => true,
+     *     'show_excerpt' => true,
+     *     'id' => 'grid-cursos'
+     * ]);
+     * ```
+     *
+     * @param \WP_Query $query Query con los posts a mostrar
+     * @param array<string, mixed> $settings Settings del componente post-grid desde BD
+     * @param array<string, mixed> $options Opciones de visualizacion del shortcode
+     *                                      Keys: columns, show_thumbnail, show_excerpt,
+     *                                      show_meta, show_categories, excerpt_length,
+     *                                      class, id, show_pagination
+     *
+     * @return string HTML completo del grid incluyendo CSS inline
+     */
+    public function render(\WP_Query $query, array $settings, array $options): string;
+}