fix: show element-specific values in commented CSS preview

- Preview now includes all field values, not filtering defaults
- Commented CSS shows what would be applied with current field values
- Applies to both ElementPopup and PagePopup

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/components/PagePopup.vue

@@ -622,9 +622,11 @@ const generatePreviewCss = () => {
 
   const properties = [];
 
+  // Always include margin with current values
   const marginValue = `${margins.value.top.value}${margins.value.top.unit} ${margins.value.right.value}${margins.value.right.unit} ${margins.value.bottom.value}${margins.value.bottom.unit} ${margins.value.left.value}${margins.value.left.unit}`;
   properties.push(`  margin: ${marginValue};`);
 
+  // Include background if it has a value
   if (background.value.value) {
     properties.push(`  background: ${background.value.value};`);
   }