fix `offsetMode` in `MkPagination` - fixes #622

What does this PR do?

3f73251d reworked the way offsetMode is handled, to allow reactive ComputedRef<boolean> in addition to a simple boolean, so that custom emoji search could easily switch between the faster sinceId/untilId pagination and the slower offset as needed.

Or it would have, if I had written the correct expression! I wrote props.offsetMode instead of props.pagination.offsetMode, so it was always false

I have no idea how I didn't notice, I swear I tested it ☹

Anyway, factoring out the nested ternaries makes the whole thing clearer and less prone to mistakes. And I have tested it, this time.

@kio could you cherry-pick this commit into your fork and test it?

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