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>

src/utils/kirby-auth.js

@@ -0,0 +1,37 @@
+/**
+ * Kirby Authentication Utilities
+ *
+ * Helper functions for authentication and CSRF token management
+ */
+
+/**
+ * Get CSRF token from meta tag
+ * @returns {string|null} CSRF token or null if not found
+ */
+export function getCsrfToken() {
+  // Check for meta tag (added by header.php when user is logged in)
+  const metaTag = document.querySelector('meta[name="csrf"]');
+  if (metaTag) {
+    return metaTag.getAttribute('content');
+  }
+
+  // Alternatively, could be in a cookie
+  const cookies = document.cookie.split(';');
+  for (let cookie of cookies) {
+    const [name, value] = cookie.trim().split('=');
+    if (name === 'kirby_csrf') {
+      return decodeURIComponent(value);
+    }
+  }
+
+  return null;
+}
+
+/**
+ * Check if user is authenticated (has Kirby session)
+ * @returns {boolean} True if authenticated, false otherwise
+ */
+export function isAuthenticated() {
+  // Check for kirby session cookie
+  return document.cookie.includes('kirby_session');
+}