Move project save and open into a toolbar menu, and surface messages in one place

The two project actions sit behind a folder button beside "clear canvas", as
icons only. The saved file is named m3e-canvas followed by the app's name. On
the desktop every short message (applied, copied, an unreadable file, an AI
error) now appears in the pill beside the tidy button; the phone keeps its
centered toast.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>