feat: add custom CSS save system with dual-editor interface

Implement complete custom CSS management system:
- Separate base CSS (readonly) and custom CSS (editable)
- Save custom CSS to Kirby backend per narrative
- Visual save button with state indicators (dirty/saving/success/error)
- CSRF-protected API endpoint for CSS operations
- Dual-editor StylesheetViewer (base + custom with edit mode toggle)
- Auto-format custom CSS with Prettier on edit mode exit

Backend changes:
- Add web2print Kirby plugin with POST/GET routes
- Add customCss field to narrative blueprint
- Add CSRF token meta tag in header
- Include customCss and modified timestamps in JSON template
- Install code-editor plugin for Kirby panel

Frontend changes:
- Refactor stylesheet store with baseCss/customCss refs
- Make content a computed property (baseCss + customCss)
- Add helper methods: replaceBlock, replaceInCustomCss, setCustomCss
- Update all components to use new store API
- Create SaveButton component with FAB design
- Redesign StylesheetViewer with collapsable sections
- Initialize store from narrative data on app mount

File changes:
- Rename stylesheet.css → stylesheet.print.css
- Update all references to new filename

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

public/vendor/composer/installed.php

@@ -3,7 +3,7 @@
         'name' => 'getkirby/plainkit',
         'pretty_version' => 'dev-main',
         'version' => 'dev-main',
-        'reference' => '76274fff04c54514230ad2bb0aca362139618411',
+        'reference' => '4d1183d1afd90517610e742ea0bc6553e8312bc6',
         'type' => 'project',
         'install_path' => __DIR__ . '/../../',
         'aliases' => array(),
@@ -67,7 +67,7 @@
         'getkirby/plainkit' => array(
             'pretty_version' => 'dev-main',
             'version' => 'dev-main',
-            'reference' => '76274fff04c54514230ad2bb0aca362139618411',
+            'reference' => '4d1183d1afd90517610e742ea0bc6553e8312bc6',
             'type' => 'project',
             'install_path' => __DIR__ . '/../../',
             'aliases' => array(),
@@ -124,6 +124,15 @@
             'aliases' => array(),
             'dev_requirement' => false,
         ),
+        'sylvainjule/code-editor' => array(
+            'pretty_version' => '1.1.0',
+            'version' => '1.1.0.0',
+            'reference' => 'adbc2c8a728994cc57ea72a7f8628f27d202b8df',
+            'type' => 'kirby-plugin',
+            'install_path' => __DIR__ . '/../../site/plugins/code-editor',
+            'aliases' => array(),
+            'dev_requirement' => false,
+        ),
         'symfony/deprecation-contracts' => array(
             'pretty_version' => 'v3.6.0',
             'version' => '3.6.0.0',