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/site/plugins/code-editor/lib/fields/code-editor.php

@@ -0,0 +1,32 @@
+<?php
+
+$options = require kirby()->root('kirby') . '/config/fields/textarea.php';
+
+/* Merge new properties
+--------------------------------*/
+
+$options = A::merge($options, [
+    'props' => [
+        'size'         => function($size = null) {
+    		return $size ?? option('sylvainjule.code-editor.size');
+        },
+        'language'     => function($language = null) {
+            return $language ?? option('sylvainjule.code-editor.language');
+        },
+        'lineNumbers'  => function($lineNumbers = null) {
+            return $lineNumbers ?? option('sylvainjule.code-editor.lineNumbers');
+        },
+        'tabSize'      => function($tabSize = null) {
+            return $tabSize ?? option('sylvainjule.code-editor.tabSize');
+        },
+        'insertSpaces' => function($insertSpaces = null) {
+            return $tabSize ?? option('sylvainjule.code-editor.insertSpaces');
+        },
+        'ignoreTabKey' => function($ignoreTabKey = null) {
+            return $tabSize ?? option('sylvainjule.code-editor.ignoreTabKey');
+        },
+    ]
+]);
+
+// return the updated options
+return $options;