feat: add purple highlight for content elements

- Add --color-purple variable (#7136ff)
- Hover: dashed purple outline on content elements
- Selected: purple background with 30% opacity
- ElementPopup: purple label and instance count
- Track hovered and selected elements separately from pages
- Clear element selection when popup closes

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

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

public/assets/css/src/_variables.scss

@@ -5,6 +5,7 @@
   --color-browngray-300: #b5a9a1;
 
   --color-page-highlight: #ff8a50;
+  --color-purple: #7136ff;
 
   --border-radius: 0.2rem;