Open and save projects through the app's own dialog, toast and restore path

Follows up on #5. The file check and the download move to lib/project.ts, the
"replace the canvas?" question uses ConfirmDialog and an unreadable file the
toast, and opening a file goes through the same applyDoc the saved document
goes through, so the migrations run in one place. The file is named after the
app's title.

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