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/package.json

@@ -0,0 +1,30 @@
+{
+	"name": "kirby-code-editor",
+	"version": "1.0.3",
+	"description": "Code editor field for Kirby 3 and 4",
+	"main": "index.js",
+	"author": "Kirby Community",
+	"license": "MIT",
+	"repository": {
+		"type": "git",
+		"url": "git@github.com:sylvainjule/kirby-code-editor.git"
+	},
+	"scripts": {
+		"dev": "kirbyup src/index.js --watch",
+		"build": "kirbyup src/index.js",
+		"lint": "eslint \"src/**/*.{js,vue}\"",
+		"lint:fix": "npm run lint -- --fix",
+		"format": "prettier --write \"src/**/*.{css,js,vue}\"",
+		"prepare": "node src/node/patchVuePrismEditor.mjs"
+	},
+	"devDependencies": {
+		"consola": "^3.4.2",
+		"eslint": "^9.27.0",
+		"eslint-config-prettier": "^10.1.5",
+		"eslint-plugin-vue": "^9.33.0",
+		"kirbyup": "^3.3.0",
+		"prettier": "^3.5.3",
+		"prismjs": "^1.30.0",
+		"vue-prism-editor": "^1.3.0"
+	}
+}