refactor(inc, image-optimizer): restructure /inc architecture and standardize image optimizer module

- reorganized /inc directory structure for improved separation of concerns
  - grouped files into customizer, utilities, and tools
  - improved naming consistency across files (e.g. *-settings, template-tags, etc.)
  - simplified functions.php includes for better readability and maintainability

- refactored Customizer structure
  - extracted image optimizer settings from core-settings into dedicated module
  - consolidated settings, UI logic, and styles into a single feature file
  - improved naming consistency for hooks and functions

- standardized Image Optimizer admin tool
  - renamed "Performance Tools" to "Image Optimizer" across UI and hooks
  - updated admin page registration, callback names, and menu labels
  - aligned AJAX nonce naming for consistency and clarity

- preserved all existing logic and behavior (no functional changes)
- improved overall code organization and long-term maintainability

no breaking changes

inc/utilities/template-functions.php

@@ -0,0 +1,37 @@
+<?php
+/**
+ * Functions which enhance the theme by hooking into WordPress
+ *
+ * @package zeitfresser
+ */
+
+/**
+ * Adds custom classes to the array of body classes.
+ *
+ * @param array $classes Classes for the body element.
+ * @return array
+ */
+function zeitfresser_body_classes( $classes ) {
+	// Adds a class of hfeed to non-singular pages.
+	if ( ! is_singular() ) {
+		$classes[] = 'hfeed';
+	}
+
+	// Adds a class of no-sidebar when there is no sidebar present.
+	if ( ! is_active_sidebar( 'sidebar-1' ) ) {
+		$classes[] = 'no-sidebar';
+	}
+
+	return $classes;
+}
+add_filter( 'body_class', 'zeitfresser_body_classes' );
+
+/**
+ * Add a pingback url auto-discovery header for single posts, pages, or attachments.
+ */
+function zeitfresser_pingback_header() {
+	if ( is_singular() && pings_open() ) {
+		printf( '<link rel="pingback" href="%s">', esc_url( get_bloginfo( 'pingback_url' ) ) );
+	}
+}
+add_action( 'wp_head', 'zeitfresser_pingback_header' );