fix: resize handle fills full panel height via self-stretch

The ResizeHandle div was not stretching to the full height of its flex
row parent, making it only interactive in the top portion. Adding
self-stretch (align-self: stretch) ensures it fills the complete cross-axis
height of the container, so the user can drag at any vertical position.

Fixes: Properties panel resize handle active area.

src/components/ResizeHandle.tsx

@@ -67,7 +67,7 @@ export function ResizeHandle({ onResize }: ResizeHandleProps) {
 
   return (
     <div
-      className="-ml-1 w-1 shrink-0 cursor-col-resize bg-transparent transition-colors hover:bg-[var(--border)]"
+      className="-ml-1 w-1 shrink-0 self-stretch cursor-col-resize bg-transparent transition-colors hover:bg-[var(--border)]"
       onMouseDown={handleMouseDown}
     />
   )