docs(window-switcher): an empty state, rather than opening on nothing

ALT + TAB with no windows would otherwise give a full-screen dim with
nothing in it, which reads as a hang. It is reachable from a fresh
session, and from closing the last window in the switcher itself, which
stays open by design. It now says so instead.

The empty state has no card to click, so the backdrop closes the overlay
on click as well as on Escape. That applies generally, not only when
there is nothing to show.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SYg4wYHq5XNbiVmMeKRb1S

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