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/social.php

@@ -0,0 +1,81 @@
+<?php
+/**
+ * Social Links Customizer Options
+ *
+ * @package zeitfresser
+ */
+
+if ( ! function_exists( 'zeitfresser_get_social_links' ) ) {
+    /**
+     * Return supported social networks.
+     *
+     * @return array<string,string>
+     */
+    function zeitfresser_get_social_links() {
+        return array(
+            'facebook'  => esc_html__( 'Facebook', 'zeitfresser' ),
+            'instagram' => esc_html__( 'Instagram', 'zeitfresser' ),
+            'youtube'   => esc_html__( 'YouTube', 'zeitfresser' ),
+            'linkedin'  => esc_html__( 'LinkedIn', 'zeitfresser' ),
+            'twitter'   => esc_html__( 'Twitter', 'zeitfresser' ),
+            'pinterest' => esc_html__( 'Pinterest', 'zeitfresser' ),
+            'tiktok'    => esc_html__( 'TikTok', 'zeitfresser' ),
+            'mastodon'  => esc_html__( 'Mastodon', 'zeitfresser' ),
+            'github'    => esc_html__( 'GitHub', 'zeitfresser' ),
+            'matrix'    => esc_html__( 'Matrix.org', 'zeitfresser' ),
+        );
+    }
+}
+
+add_action( 'customize_register', 'zeitfresser_social_links' );
+
+function zeitfresser_social_links( $wp_customize ) {
+
+    $social_links = zeitfresser_get_social_links();
+
+    /**
+     * Section Divider
+     */
+    $wp_customize->add_setting(
+        'ztfr_social_heading',
+        array(
+            'sanitize_callback' => 'wp_kses_post',
+        )
+    );
+
+    $wp_customize->add_control(
+        'ztfr_social_heading',
+        array(
+            'section'     => 'ztfr_general',
+            'type'        => 'hidden',
+            'description' => '<hr><strong>' . esc_html__( 'Social Links', 'zeitfresser' ) . '</strong>',
+            'priority'    => 30,
+        )
+    );
+
+    /**
+     * Social URLs
+     */
+    $priority = 31;
+
+    foreach ( $social_links as $key => $label ) {
+
+        $wp_customize->add_setting(
+            'social_links_' . $key,
+            array(
+                'default'           => '',
+                'sanitize_callback' => 'esc_url_raw',
+            )
+        );
+
+        $wp_customize->add_control(
+            'social_links_' . $key,
+            array(
+                'type'     => 'url',
+                'section'  => 'ztfr_general',
+                'label'    => $label,
+                'priority' => $priority++,
+            )
+        );
+    }
+}