chore(sidebar): document labels menu invariants and remove dead code (#707)

Document the hidden constraints that PR #706 left in the codebase so they
are not accidentally removed in a future cleanup pass:

- DropdownMenuSubContent and ContextMenuSubContent must stay Portal-wrapped
  so sub-menus escape ancestors with overflow-x-hidden.
- refreshLabels must stay stable via useCallback because it is captured by
  buildMenuCtx's memoization and passed as a prop.

Also delete LabelAssignPopover.tsx, which had zero consumers after the
create-and-assign flow moved into the menu-layer inline form.

frontend/src/components/EditorLayout.tsx

@@ -597,6 +597,7 @@ export default function EditorLayout() {
     setStackStatuses(prev => ({ ...prev, [stackFile]: status }));
   };
 
+  // Stable identity required: captured by buildMenuCtx's memoization and passed as a prop; unstable refs cause descendant re-render churn.
   const refreshLabels = useCallback(async () => {
     if (!isPaid) return;
     try {