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>

functions-addon.php

@@ -562,3 +562,22 @@ function roi_get_adsense_search_config(): array {
         ],
     ];
 }
+
+// =============================================================================
+// POST GRID SHORTCODE [roi_post_grid]
+// =============================================================================
+
+/**
+ * Registra el shortcode [roi_post_grid] para mostrar grids de posts
+ * en cualquier pagina o entrada.
+ *
+ * USO:
+ * [roi_post_grid]
+ * [roi_post_grid category="precios-unitarios" posts_per_page="6"]
+ * [roi_post_grid id="grid-1" category="cursos" show_pagination="true"]
+ *
+ * @see Shared/Infrastructure/Wordpress/PostGridShortcodeRegistrar
+ */
+add_action('init', function() {
+    \ROITheme\Shared\Infrastructure\Wordpress\PostGridShortcodeRegistrar::register();
+});