Sync single-file mode user preference

What does this MR do?

For #284116 (closed)

MR	Description
!55033 (merged)	Exposes the endpoint to update the user settings
!55930 (merged)	Update the Diffs app to ingest and store the endpoint that we will use to update the user preference for file-by-file in the future
We're here 👉🏻	Update the Diffs app to persist the user's Single File preference to their account when they change it from the ⚙ dropdown menu

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

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