Refactor ToggleSwitch knob to DRY up 1px/2px values via custom properties

Introduce --toggleSwitch-knob-inset (the track border width) and
--toggleSwitch-knob-offset (twice the inset), rooting the knob inset,
checked travel, and concentric border-radius in a single source of truth.
No visual change.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: dylanatsmith

app/components/primer/alpha/toggle_switch.pcss

@@ -7,6 +7,10 @@
 }
 
 .ToggleSwitch {
+  /* Knob clears the track's border on every side */
+  --toggleSwitch-knob-inset: var(--borderWidth-thin);
+  --toggleSwitch-knob-offset: calc(2 * var(--toggleSwitch-knob-inset));
+
   align-items: center;
   display: inline-flex;
   gap: var(--controlStack-medium-gap-condensed);
@@ -93,7 +97,7 @@
   & .ToggleSwitch-knob {
     background-color: var(--controlKnob-bgColor-checked);
     border-color: var(--controlKnob-borderColor-checked);
-    transform: translateX(calc(100% - 2px));
+    transform: translateX(calc(100% - var(--toggleSwitch-knob-offset)));
   }
 
   & .ToggleSwitch-lineIcon {
@@ -155,18 +159,18 @@
 .ToggleSwitch-knob {
   position: absolute;
   /* stylelint-disable-next-line primer/spacing */
-  top: 1px;
+  top: var(--toggleSwitch-knob-inset);
   /* stylelint-disable-next-line primer/spacing */
-  bottom: 1px;
+  bottom: var(--toggleSwitch-knob-inset);
   /* stylelint-disable-next-line primer/spacing */
-  left: 1px;
+  left: var(--toggleSwitch-knob-inset);
   z-index: 1;
   width: 50%;
   background-color: var(--controlKnob-bgColor-rest);
   border: var(--borderWidth-thin) solid var(--controlKnob-borderColor-rest);
-  /* Use calc to account for the 1px border around the track */
+  /* Concentric radius: subtract the track border on both sides */
   /* stylelint-disable-next-line primer/borders */
-  border-radius: calc(var(--borderRadius-medium) - var(--borderWidth-thick));
+  border-radius: calc(var(--borderRadius-medium) - var(--toggleSwitch-knob-offset));
   transition-timing-function: cubic-bezier(0.5, 1, 0.89, 1);
   transition-duration: 80ms;
   transition-property: transform;