refactor(pins): drop the unused ComfortPinMenu and the comment that described it

Every surface composes the pin row into its object's one action list (E28 C1); the pin-only menu
had no caller left. Also un-exports isStandInFocus, which only its own file reads since H1.

frontend/src/lib/useFocusKeeper.ts

@@ -52,7 +52,7 @@ export function focusNeighbour(el: HTMLElement): boolean {
 let standIn: HTMLElement | null = null;
 
 /** Whether `el` holds the focus only because the keeper put it there. */
-export function isStandInFocus(el: Element | null): boolean {
+function isStandInFocus(el: Element | null): boolean {
   return el !== null && el === standIn;
 }