buildFlashCookie(id: string,message: string,succeeded: boolean,result?: string,level?: FlashLevel,formToken?: string,): string
Build a flash cookie containing a success, error, or info message, keyed by
ID. level defaults to success/error from succeeded; pass it to override.
formToken redeems the in-memory form re-fill stash on the follow-up GET; it
rides in this HttpOnly, SameSite=Strict cookie rather than the URL so it
can't be guessed or leaked via history/referrer.
optional
level: FlashLevel