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/customizer/toc-settings.php

@@ -0,0 +1,101 @@
+<?php
+/**
+ * TOC Customizer Options
+ *
+ * @package zeitfresser
+ */
+
+add_action( 'customize_register', 'zeitfresser_toc_options' );
+
+function zeitfresser_toc_options( $wp_customize ) {
+
+    /**
+     * 🔥 Heading Control (falls noch nicht vorhanden)
+     */
+    if ( class_exists( 'WP_Customize_Control' ) && ! class_exists( 'ZTFR_Customize_Heading_Control' ) ) {
+        class ZTFR_Customize_Heading_Control extends WP_Customize_Control {
+            public $type = 'ztfr-heading';
+
+            public function render_content() {
+                ?>
+                <span style="display:block; font-weight:600; font-size:14px; margin:15px 0 5px;">
+                    <?php echo esc_html( $this->label ); ?>
+                </span>
+                <?php
+            }
+        }
+    }
+
+    /**
+     * ------------------------
+     * TOC HEADING
+     * ------------------------
+     */
+    $wp_customize->add_setting(
+        'ztfr_toc_heading',
+        array(
+            'sanitize_callback' => 'sanitize_text_field',
+        )
+    );
+
+    $wp_customize->add_control(
+        new ZTFR_Customize_Heading_Control(
+            $wp_customize,
+            'ztfr_toc_heading',
+            array(
+                'label'    => esc_html__( 'TOC', 'zeitfresser' ),
+                'section'  => 'ztfr_general',
+                'priority' => 10,
+            )
+        )
+    );
+
+    /**
+     * Show TOC
+     */
+    $wp_customize->add_setting(
+        'show_article_toc',
+        array(
+            'default'           => true,
+            'sanitize_callback' => 'wp_validate_boolean',
+        )
+    );
+
+    $wp_customize->add_control(
+        'show_article_toc',
+        array(
+            'type'        => 'checkbox',
+            'section'     => 'ztfr_general',
+            'label'       => esc_html__( 'Show Article TOC', 'zeitfresser' ),
+            'description' => esc_html__( 'Enable floating TOC on single posts.', 'zeitfresser' ),
+            'priority'    => 11,
+        )
+    );
+
+    /**
+     * Minimum headlines threshold
+     */
+    $wp_customize->add_setting(
+        'article_toc_min_headlines',
+        array(
+            'default'           => 3,
+            'sanitize_callback' => 'absint',
+        )
+    );
+
+    $wp_customize->add_control(
+        'article_toc_min_headlines',
+        array(
+            'type'        => 'number',
+            'section'     => 'ztfr_general',
+            'label'       => esc_html__( 'Minimum Headlines', 'zeitfresser' ),
+            'description' => esc_html__( 'TOC appears only if this number of headings is reached.', 'zeitfresser' ),
+            'priority'    => 12,
+            'input_attrs' => array(
+                'min'  => 1,
+                'max'  => 50,
+                'step' => 1,
+            ),
+        )
+    );
+}