feat: implement reactive EditorPanel with bidirectional sync

- Reorganize editor components into dedicated folder
- Create PageSettings component with page format, margins, background controls
- Create TextSettings component (structure only, to be populated)
- Implement debounced updates (1s delay) to stylesheet store
- Add bidirectional sync between EditorPanel and StylesheetViewer
- Preserve scroll position as percentage when reloading preview
- Move @page rules from App.vue to stylesheet.css for unified management
- Extend css-parsing utils to handle text values (e.g., 'A4', 'portrait')
- Remove unnecessary comments, use explicit naming instead

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

public/assets/css/stylesheet.css

@@ -1,3 +1,20 @@
+@page {
+  size: A4;
+  margin: 20mm 15mm 26mm 15mm;
+}
+
+@page {
+  @bottom-center { content: string(title); }
+}
+
+h2 {
+  break-before: page;
+}
+
+.chapter > h2 {
+  string-set: title content(text);
+}
+
 #chapter-2 {
   font-size: 2rem;
 }