fix: honor an explicit zero rotation origin for keys (#175)

Keys with a rotation (r != 0) whose origin rx/ry is explicitly 0 were
pivoted around each key's own corner instead of the layout origin (0,0),
scrambling rotated clusters on boards that rotate about the origin.

`(rx || x)` collapsed a legitimate 0 back to the key's own x; switching to
`(rx ?? x)` keeps an explicit 0 as 0.

Fixes #97

Co-authored-by: Claude <noreply@anthropic.com>

Author: numachang

src/keyboard/PhysicalLayout.tsx

@@ -56,8 +56,11 @@ function scalePosition(
   const transformStyle = "preserve-3d";
 
   if (r) {
-    let transformX = ((rx || x) - x) * oneU;
-    let transformY = ((ry || y) - y) * oneU;
+    // Use `??` so an explicit rotation origin of 0 is honored; `rx || x`
+    // collapsed a legitimate 0 back to the key's own position, pivoting the
+    // key around its own corner instead of the layout origin (#97).
+    let transformX = ((rx ?? x) - x) * oneU;
+    let transformY = ((ry ?? y) - y) * oneU;
     transformOrigin = `${transformX}px ${transformY}px`;
     transform = `rotate(${r}deg)`;
   }