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/composer.json

@@ -0,0 +1,20 @@
+{
+	"name": "sylvainjule/code-editor",
+	"description": "Code editor field for Kirby 3, 4 and 5",
+	"type": "kirby-plugin",
+	"license": "MIT",
+	"version": "1.1.0",
+	"authors": [
+		{
+			"name": "Sylvain Julé",
+			"email": "contact@sylvain-jule.fr"
+		}
+	],
+	"require": {
+		"getkirby/composer-installer": "^1.2"
+	},
+	"extra": {
+		"installer-name": "code-editor"
+	},
+	"minimum-stability": "beta"
+}