_showDiagnosis referenced an undefined `body` (left over from the refactor
that moved the diagnosis text into the toolbar), throwing a ReferenceError
whenever a failed task rendered fix buttons. Because open() wraps its render
in try/finally with no catch, the throw escaped before the modal was
un-hidden, so the whole Cookbook silently failed to open.
- cookbook-diagnosis.js: append the fixes row to `diag` (the in-scope
container) instead of the removed `body` element.
- cookbook.js: guard the render passes in open() so one broken task card
can't leave the entire panel stuck hidden.
Fixes#4406
497f455da6