Track dirty from Craft node changes and page/design/header/footer state, cleared
only by a successful save. Persistent Unsaved/Saving/Saved/Published/Save failed
pill, beforeunload prompt while dirty or saving, one persistent error notice,
resp.ok/non-JSON handling for save and publish, and publish refuses to run
after a failed save. Draft badge no longer flips on a no-op autosave.
Refs #30
Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>