Function
The raw string from storage, or null when there is none
One of the offered steps, in seconds
Total by design: this reads a value the learner's browser may have had for months, that another tab may have written, or that a hand in DevTools may have mangled. Anything that is not one of SEEK_STEP_OPTIONS_SECONDS spelled exactly — an absent entry, a number outside the set (an interval this app no longer offers included), a float, a string with units — means "never chosen" and resolves to DEFAULT_SEEK_STEP_SECONDS. Nothing here throws, because failing to remember a preference is not worth breaking the lesson the learner is on.
Resolves a stored seek step to one the player can use.