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/web2print/index.php

@@ -0,0 +1,116 @@
+<?php
+/**
+ * Web2Print Plugin
+ *
+ * Routes for web-to-print functionality including custom CSS management
+ */
+
+use Kirby\Cms\Response;
+
+Kirby::plugin('geoproject/web2print', [
+    'routes' => [
+        // POST: Save custom CSS
+        [
+            'pattern' => 'narratives/(:all)/css',
+            'method' => 'POST',
+            'action' => function ($pagePath) {
+                // Check authentication
+                if (!kirby()->user()) {
+                    return Response::json([
+                        'status' => 'error',
+                        'message' => 'Authentication required'
+                    ], 401);
+                }
+
+                // Verify CSRF token from header
+                $csrfToken = kirby()->request()->header('X-CSRF');
+                if (!csrf($csrfToken)) {
+                    return Response::json([
+                        'status' => 'error',
+                        'message' => 'Invalid CSRF token'
+                    ], 403);
+                }
+
+                // Get page
+                $page = page($pagePath);
+
+                if (!$page || $page->intendedTemplate()->name() !== 'narrative') {
+                    return Response::json([
+                        'status' => 'error',
+                        'message' => 'Narrative not found'
+                    ], 404);
+                }
+
+                // Get POST data
+                $data = kirby()->request()->data();
+                $customCss = $data['customCss'] ?? null;
+
+                if ($customCss === null) {
+                    return Response::json([
+                        'status' => 'error',
+                        'message' => 'No CSS content provided'
+                    ], 400);
+                }
+
+                try {
+                    // Update page with new custom CSS
+                    $page->update([
+                        'customCss' => $customCss
+                    ]);
+
+                    // Reload page to get updated modification time
+                    $page = page($pagePath);
+
+                    // Return success with updated modified timestamp
+                    return Response::json([
+                        'status' => 'success',
+                        'data' => [
+                            'modified' => $page->modified(),
+                            'modifiedFormatted' => $page->modified('d/m/Y H:i')
+                        ]
+                    ]);
+                } catch (Exception $e) {
+                    return Response::json([
+                        'status' => 'error',
+                        'message' => 'Failed to save CSS: ' . $e->getMessage()
+                    ], 500);
+                }
+            }
+        ],
+
+        // GET: Load custom CSS and last modified time
+        [
+            'pattern' => 'narratives/(:all)/css',
+            'method' => 'GET',
+            'action' => function ($pagePath) {
+                // Check authentication
+                if (!kirby()->user()) {
+                    return Response::json([
+                        'status' => 'error',
+                        'message' => 'Authentication required'
+                    ], 401);
+                }
+
+                // Get page
+                $page = page($pagePath);
+
+                if (!$page || $page->intendedTemplate()->name() !== 'narrative') {
+                    return Response::json([
+                        'status' => 'error',
+                        'message' => 'Narrative not found'
+                    ], 404);
+                }
+
+                // Return custom CSS content and modified timestamp
+                return Response::json([
+                    'status' => 'success',
+                    'data' => [
+                        'customCss' => $page->customCss()->value() ?? '',
+                        'modified' => $page->modified(),
+                        'modifiedFormatted' => $page->modified('d/m/Y H:i')
+                    ]
+                ]);
+            }
+        ]
+    ]
+]);