gui: Sharpen device icons (fixes #5579) (#6837)

Add [shape-rendering: crispEdges](https://developer.mozilla.org/en-US/docs/Web/SVG/Attribute/shape-rendering) to `.identicon` class to remove blurry grid lines within device icons

gui/default/assets/css/overrides.css

@@ -241,6 +241,7 @@ identicon {
 .identicon {
     width: 1em;
     height: 1em;
+    shape-rendering: crispEdges;
 }
 
 a.toggler {