Is it permitted to prepare things on Shabbat for after Shabbat?
No. There is a prohibition on preparing from the sacred (Shabbat) for the mundane (weekday)
Source: Shulchan Aruch OC 290
In detail
It is forbidden to prepare on Shabbat for after Shabbat (hachana). This includes washing dishes needed only for Saturday night, setting the table for a post-Shabbat event, or packing a bag for a trip after Shabbat. The principle is that Shabbat's time and energy must be dedicated to Shabbat itself. This also means you shouldn't say 'after Shabbat I'll do X' in a planning manner.
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