Undo deleted time entry within ten seconds

Add an undo for deleting a time entry, offered for ten seconds after the delete.

Deleting an entry now offers Undo in the toast for 10s; it restores the entry in place (restoreEntry + 4 tests). Gate green; checked in a browser at 1280/390px. PR #70 merged. Follow-up: the mobile entry rows already overflow on develop.