fix: add z-index to handles so they paint above positioned node content

Nodes with `position: relative` on inner wrappers (e.g. image wrapper
needing it for its remove button) caused left-side handles to render
behind the wrapper due to CSS painting order. Adding z-index: 5 to
all handles ensures they always paint above node content.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/app/globals.css

@@ -79,6 +79,7 @@ body {
   border-radius: 50%;
   border: 2px solid white;
   box-shadow: 0 1px 3px rgba(0, 0, 0, 0.15);
+  z-index: 5;
 }
 
 /* Larger invisible hit area for easier clicking, especially when zoomed out */