fix(dashboard): correct rem-to-pixel conversion for drag overlay grid cells

- Convert rowHeight and gap from rem to pixels using gridEngine.remToPixels()
- Fix highlightGridCells() to use pixel values for positioning and sizing
- Fix showGridOverlay() to calculate container height in pixels
- Resolves issue where green highlight boxes rendered as tiny squished lines
- Follows same conversion pattern used in gridEngine.getPixelPosition()

style.css

@@ -1158,6 +1158,7 @@ body:has(.rpg-panel.rpg-position-left) #sheld {
     min-height: 0; /* Allow flex to shrink below natural size */
 }
 
+
 /* Hide resize handles by default */
 .resize-handles {
     opacity: 0;