feat(buttons): add ToggleButton component

Keep toggle styling and the accessible pressed state coupled in one reusable button.

apps/client/src/common/components/buttons/ToggleButton.tsx

@@ -0,0 +1,18 @@
+import { ComponentProps } from 'react';
+
+import Button from './Button';
+
+type ToggleButtonProps = Omit<ComponentProps<typeof Button>, 'variant'> & {
+  /** whether the option this button controls is currently on */
+  pressed: boolean;
+};
+
+/**
+ * A button which carries an on / off state.
+ *
+ * Keeps the pressed styling and the accessible state together, so that a toggle
+ * cannot end up looking active without also announcing that it is.
+ */
+export default function ToggleButton({ pressed, ...buttonProps }: ToggleButtonProps) {
+  return <Button variant={pressed ? 'primary' : 'subtle'} aria-pressed={pressed} {...buttonProps} />;
+}