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/ui/context-menu.tsx

@@ -37,6 +37,7 @@ const ContextMenuSubTrigger = React.forwardRef<
 ))
 ContextMenuSubTrigger.displayName = ContextMenuPrimitive.SubTrigger.displayName
 
+// Portal-wrapped so sub-menus escape ancestors with overflow-x-hidden. Keep the Portal.
 const ContextMenuSubContent = React.forwardRef<
   React.ElementRef<typeof ContextMenuPrimitive.SubContent>,
   React.ComponentPropsWithoutRef<typeof ContextMenuPrimitive.SubContent>

frontend/src/components/ui/dropdown-menu.tsx

@@ -38,6 +38,7 @@ const DropdownMenuSubTrigger = React.forwardRef<
 DropdownMenuSubTrigger.displayName =
   DropdownMenuPrimitive.SubTrigger.displayName
 
+// Portal-wrapped so sub-menus escape ancestors with overflow-x-hidden (e.g. the sidebar kebab container). Keep the Portal.
 const DropdownMenuSubContent = React.forwardRef<
   React.ElementRef<typeof DropdownMenuPrimitive.SubContent>,
   React.ComponentPropsWithoutRef<typeof DropdownMenuPrimitive.SubContent>