menu: Fix dangling focus after `ContextMenu` dismissed (#2614)

## Description

The fallback element ID was derived from a stack address, which changes
when gpui redraws from the event-dispatch path, silently dropping the
element state (open menu) without emitting `DismissEvent`. Use the
caller location as ID instead, and pass the previously focused handle as
`action_context` so `PopupMenu` restores focus on dismiss.

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>