Fix drag-and-drop hover detection and slot targeting

Two root-cause bugs fixed:

1. Port <button> elements inside ModuleBlock had pointer-events:auto (browser
   default), so document.elementFromPoint() hit them instead of the RackSlot
   behind them whenever the cursor was over an occupied slot. Fixed by toggling
   body.rack-dragging during any drag, which applies a CSS rule that forces
   pointer-events:none !important on .module-block and all descendants.

2. onDragMove pointer-position reconstruction (activatorEvent.clientX + delta.x)
   was slightly off because delta is measured from the initial mousedown, not
   the activation point. Replaced with a native window pointermove listener
   (capture phase) that gives exact clientX/Y — no reconstruction needed.

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

client/src/components/rack/ModuleBlock.tsx

@@ -116,7 +116,7 @@ export function ModuleBlock({ module }: ModuleBlockProps) {
         {...listeners}
         {...attributes}
         className={cn(
-          'relative w-full border-l-4 select-none overflow-hidden transition-opacity',
+          'module-block relative w-full border-l-4 select-none overflow-hidden transition-opacity',
           colors.bg,
           colors.border,
           isDragging ? 'opacity-0 pointer-events-none' : 'cursor-grab active:cursor-grabbing',