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/composables/usePopupPosition.js

@@ -0,0 +1,52 @@
+export function usePopupPosition(popupWidth, popupHeight) {
+  const OFFSET = 10; // Distance from cursor
+
+  const calculatePosition = (event) => {
+    // Get cursor position relative to viewport
+    const cursorX = event.clientX;
+    const cursorY = event.clientY;
+
+    const viewportWidth = window.innerWidth;
+    const viewportHeight = window.innerHeight;
+
+    let x, y;
+
+    // Try bottom-right
+    if (
+      cursorX + OFFSET + popupWidth <= viewportWidth &&
+      cursorY + OFFSET + popupHeight <= viewportHeight
+    ) {
+      x = cursorX + OFFSET;
+      y = cursorY + OFFSET;
+    }
+    // Try bottom-left
+    else if (
+      cursorX - OFFSET - popupWidth >= 0 &&
+      cursorY + OFFSET + popupHeight <= viewportHeight
+    ) {
+      x = cursorX - OFFSET - popupWidth;
+      y = cursorY + OFFSET;
+    }
+    // Try top-right
+    else if (
+      cursorX + OFFSET + popupWidth <= viewportWidth &&
+      cursorY - OFFSET - popupHeight >= 0
+    ) {
+      x = cursorX + OFFSET;
+      y = cursorY - OFFSET - popupHeight;
+    }
+    // Try top-left (fallback)
+    else {
+      x = cursorX - OFFSET - popupWidth;
+      y = cursorY - OFFSET - popupHeight;
+
+      // Ensure it doesn't go off-screen
+      x = Math.max(10, x);
+      y = Math.max(10, y);
+    }
+
+    return { x, y };
+  };
+
+  return { calculatePosition };
+}