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/components/ElementPopup.vue

@@ -435,7 +435,7 @@ const removeElementBlock = () => {
     // Escape special regex characters in selector
     const escaped = selector.value.replace(/[.*+?^${}()|[\]\\]/g, '\\$&');
     // Remove the block and any surrounding whitespace
-    stylesheetStore.content = stylesheetStore.content.replace(
+    stylesheetStore.replaceInCustomCss(
       new RegExp(`\\n?${escaped}\\s*\\{[^}]*\\}\\n?`),
       '\n'
     );
@@ -581,7 +581,7 @@ const handleCssInput = (event) => {
   cssDebounceTimer = setTimeout(() => {
     const oldBlock = elementCss.value;
     if (oldBlock) {
-      stylesheetStore.content = stylesheetStore.content.replace(oldBlock, newCss);
+      stylesheetStore.replaceInCustomCss(oldBlock, newCss);
     }
   }, 500);
 };
@@ -592,7 +592,7 @@ watch(isEditable, async (newValue, oldValue) => {
 
   // Format when exiting editing mode
   if (oldValue && !newValue) {
-    await stylesheetStore.formatContent();
+    await stylesheetStore.formatCustomCss();
   }
 });