Review round 2 found restoring the page in place still loses or corrupts
work: the purge of its rows runs after the page is gone, so restored ids
can be refused while the page reads as saved; lineup screenshots were
not re-uploaded; page metadata queued before the deletion was dropped.
Each is a guess the client cannot verify, and AGENTS.md says to fail
loudly rather than save something wrong. The dialog now tells the user
their latest changes could not be saved, and the canvas moves on once
they have read it. Keeping the work needs the server's help.

Switching pages checks the page being left first, so the notice shows
even when a switch comes before anything else asked.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>