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/App.vue

@@ -4,6 +4,7 @@ import EditorPanel from './components/editor/EditorPanel.vue';
 import ElementPopup from './components/ElementPopup.vue';
 import PagePopup from './components/PagePopup.vue';
 import PreviewLoader from './components/PreviewLoader.vue';
+import SaveButton from './components/SaveButton.vue';
 import { onMounted, ref, watch, computed, provide } from 'vue';
 import { useStylesheetStore } from './stores/stylesheet';
 import { useNarrativeStore } from './stores/narrative';
@@ -554,9 +555,14 @@ const printPreview = async () => {
 };
 
 onMounted(async () => {
-  // Load narrative data if URL is provided (print mode)
+  // Load narrative data (narrativeUrl constructed from location, always present)
   await narrativeStore.loadNarrative(location.href + '.json');
 
+  // Initialize stylesheet with custom CSS
+  if (narrativeStore.data) {
+    await stylesheetStore.initializeFromNarrative(narrativeStore.data);
+  }
+
   // Render preview after data is loaded
   renderPreview(true);
 });
@@ -582,6 +588,8 @@ onMounted(async () => {
 
   <PreviewLoader :isLoading="isTransitioning" :shifted="activeTab.length > 0" />
 
+  <SaveButton />
+
   <ElementPopup
     ref="elementPopup"
     :iframeRef="activeFrame"