Push the stored preferences into the canvas document's defaults mirror.
The mirror (element_defaults / page_defaults / text_defaults) is what canvas reads at creation time, so this is the only thing that makes a stored preference do anything. It is a one-way push: nothing reads back out of the mirror into storage, which is what keeps the preference the user's and not the open document's.
Absent blocks are skipped rather than sent as {}. set_*_defaults merges, so an empty object would be a no-op anyway — but skipping keeps "nothing stored" from looking like a write, and means a host store that has never been populated cannot clear anything.
Exported so the preferences dialog can put its saved values into force immediately on OK.