refactor: mutualize popup styles into shared SCSS file

Extract ~150 lines of identical CSS from ElementPopup and PagePopup into
a new _settings-popup.scss partial:
- Common popup structure (header, body, controls, CSS panel)
- Shared components (tooltips, toggle switches, inheritance button)
- CSS editor styling (readonly display, textarea)

Component-specific styles retained:
- ElementPopup: purple theme, button groups, checkboxes
- PagePopup: orange theme, margin grid layout

Reduces duplication and improves maintainability.

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

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

public/assets/css/style.scss

@@ -4,3 +4,4 @@
 @import "src/_print-styles.scss";
 @import "src/_forms.scss";
 @import "src/_buttons.scss";
+@import "src/_settings-popup.scss";