refactor: make EditorPanel and ElementPopup fully autonomous

- Move all popup logic into ElementPopup component (state, positioning, click handling)
- Make EditorPanel autonomous with direct store access
- Simplify App.vue by removing prop drilling and intermediary logic
- Update EditorPanel to control paragraph font-size instead of .about
- Fix CSS parsing: escape selectors in extractCssValue and updateCssValue
- Remove hardcoded .about references from PagedJsWrapper

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

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

public/assets/css/stylesheet.css

@@ -1,7 +1,7 @@
-.about {
-  font-size: 1rem;
-}
-
 #chapter-2 {
   font-size: 2rem;
 }
+
+p {
+  font-size: 1rem;
+}