Copy selection to clipboard after click-chain selection updates.
The scan is pinned to the active screen captured here, not resolved
inside the deferred callback. Textual routes mouse events only to the
top of the screen stack (App.on_event forwards to self.screen), so
that is necessarily where the release landed. Without pinning, a
selection left behind on the screen below a modal would be scanned too
and emit a spurious "copied" toast alongside whatever the user actually
clicked (e.g. the Debug Console thread id, which copies itself).
self.screen needs no empty-stack guard: App.on_event already
dereferenced it to forward this MouseUp, and the only code that
empties the stack runs after the message loop has stopped.
