refactor(core): introduce modular architecture and restructure theme into clean, maintainable components

This commit introduces a complete internal refactoring of the theme architecture,
transitioning from a monolithic functions.php structure to a modular, scalable system.

### Key Changes

#### 1. Modular Architecture
- Extracted core logic into dedicated modules under `/inc/`
- Introduced clear separation of concerns:
  - `helpers/` → shared utility functions
  - `performance/` → optimization and media processing logic
  - `customizer/` → fully modularized Customizer structure
  - `template-*` → presentation and template-related logic

#### 2. Customizer Refactor
- Replaced legacy monolithic customizer with modular system:
  - `core.php` → base setup
  - `general.php`, `layout.php`, `toc.php`, `social.php` → feature-based modules
- Improved maintainability and extensibility for future settings

#### 3. Performance Layer Separation
- Moved performance-related logic into `/inc/performance/`
- Clearly separated:
  - frontend performance tweaks (scripts, CSS, cleanup)
  - media optimization tools (batch processing, cleanup UI)
- Reduced coupling between UI, logic, and processing

#### 4. Media Optimization Pipeline Stabilization
- Re-aligned upload pipeline with WordPress native flow
- Restored proper use of `wp_generate_attachment_metadata`
- Ensured compatibility with:
  - format conversion (AVIF/WebP)
  - responsive image generation
  - WordPress core image handling

#### 5. Cleaner Hook Management
- Removed duplicated hooks and redundant filters
- Standardized hook priorities and responsibilities
- Ensured consistent use of feature toggles (`get_theme_mod`)

#### 6. Improved Code Quality
- Reduced function duplication
- Standardized naming conventions
- Improved readability and inline documentation
- Removed legacy patterns and implicit dependencies

#### 7. Asset Handling Improvements
- Introduced centralized asset versioning via `filemtime`
- Improved script loading strategy (defer non-critical JS)
- Cleaner enqueue structure for styles and scripts

#### 8. Foundation for Future Development
This refactor lays the groundwork for:
- easier feature expansion
- better testability
- improved debugging capabilities
- long-term maintainability

### Breaking Changes
- Internal file structure has changed significantly
- Direct modifications to old functions.php logic may no longer apply
- Customizer extensions must now hook into modular structure

---

This is a purely structural and architectural release.
No intentional changes to frontend behavior were introduced,
but the internal system is now significantly more robust and maintainable.

single.php

@@ -2,42 +2,46 @@
 /**
  * The template for displaying all single posts
  *
- * @link https://developer.wordpress.org/themes/basics/template-hierarchy/#single-post
- *
  * @package zeitfresser
  */
 
 get_header();
-?>
-<?php $show_hide_related_posts = get_theme_mod( 'post_detail_hide_show_related_articles', zeitfresser_get_default_post_detail_related_articles() ); ?>
 
+// CLEAN: no default function anymore
+$show_hide_related_posts = get_theme_mod(
+    'post_detail_hide_show_related_articles',
+    true
+);
+?>
 
 <div id="primary" class="inside-page content-area">
     <div class="container">
         <div class="main-wrapper">
-                <section class="page-section full-width-view">
-                    <div class="detail-content">
 
-                        <?php while ( have_posts() ) : the_post(); ?>
-                            <?php get_template_part( 'template-parts/content', 'single' ); ?>
-                        <?php endwhile; // End of the loop. ?>
-                        <?php the_post_navigation(); ?>
-                        <?php comments_template(); ?>
+            <section class="page-section full-width-view">
+                <div class="detail-content">
 
-                    </div><!-- /.end of deatil-content -->
+                    <?php while ( have_posts() ) : the_post(); ?>
+                        <?php get_template_part( 'template-parts/content', 'single' ); ?>
+                    <?php endwhile; ?>
 
-                    <?php
-                        if( $show_hide_related_posts ) {
-                            get_template_part('template-parts/related', 'articles');
-                        }
-                    ?>
-                </section> <!-- /.end of section -->
+                    <?php the_post_navigation(); ?>
+                    <?php comments_template(); ?>
 
-                <div class="sidebar"><?php get_sidebar();?></div>
+                </div>
+
+                <?php if ( $show_hide_related_posts ) : ?>
+                    <?php get_template_part( 'template-parts/related', 'articles' ); ?>
+                <?php endif; ?>
+
+            </section>
+
+            <div class="sidebar">
+                <?php get_sidebar(); ?>
+            </div>
 
         </div>
     </div>
 </div>
 
-    <?php
-    get_footer();
+<?php get_footer(); ?>