feat: refactor popups with shared positioning and CSS tooltips

- Add usePopupPosition composable for smart popup positioning
  (bottom-right → bottom-left → top-right → top-left fallback)
- Refactor ElementPopup with complete controls matching mockup
- Add stylesheet sync: popups initialize from and watch store changes
- Add click-to-close behavior: clicking another element closes popup
- Add CSS property tooltips on all form labels (editor panel + popups)
  with dotted underline and monospace code tooltip on hover
- Add field--view-only class and disabled attribute on locked fields

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

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

src/App.vue

@@ -86,7 +86,7 @@ const handleIframeClick = (event) => {
   // Check if clicking near a page edge
   if (hoveredPage.value) {
     event.stopPropagation();
-    pagePopup.value.open(hoveredPage.value);
+    pagePopup.value.open(hoveredPage.value, event);
     elementPopup.value.close();
     return;
   }