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.

inc/customizer/toc.php

@@ -0,0 +1,80 @@
+<?php
+/**
+ * TOC Customizer Options
+ *
+ * @package zeitfresser
+ */
+
+add_action( 'customize_register', 'zeitfresser_toc_options' );
+
+function zeitfresser_toc_options( $wp_customize ) {
+
+    /**
+     * Section Divider (UI only)
+     */
+    $wp_customize->add_setting(
+        'ztfr_toc_heading',
+        array(
+            'sanitize_callback' => 'wp_kses_post',
+        )
+    );
+
+    $wp_customize->add_control(
+        'ztfr_toc_heading',
+        array(
+            'section'     => 'ztfr_general',
+            'type'        => 'hidden',
+            'description' => '<hr><strong>' . esc_html__( 'Article TOC', 'zeitfresser' ) . '</strong>',
+            'priority'    => 20,
+        )
+    );
+
+    /**
+     * Toggle 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'    => 21,
+        )
+    );
+
+    /**
+     * 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 for TOC', 'zeitfresser' ),
+            'description' => esc_html__( 'TOC appears only if this number of headings is reached.', 'zeitfresser' ),
+            'priority'    => 22,
+            'input_attrs' => array(
+                'min'  => 1,
+                'max'  => 50,
+                'step' => 1,
+            ),
+        )
+    );
+}