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/assets/css/stylesheet.css

@@ -1,49 +0,0 @@
-@page {
-  size: A4;
-  margin: 20mm 15mm 26mm 15mm;
-  background: rgba(255, 255, 255, 1);
-}
-
-@page {
-  @bottom-center {
-    content: string(title);
-  }
-}
-
-body {
-  font-family: "DM Sans", sans-serif;
-}
-
-h1 {
-  font-size: 3rem;
-}
-
-h2 {
-  break-before: page;
-}
-
-.chapter > h2 {
-  string-set: title content(text);
-}
-
-#chapter-2 {
-  font-size: 2rem;
-}
-
-p {
-  font-size: 1rem;
-  margin: 0mm 0mm 5mm 0mm;
-}
-
-li p {
-  margin-bottom: 0;
-}
-
-img {
-  width: 100%;
-}
-
-a {
-  color: #7136ff;
-  text-decoration: none;
-}