Fix: moving a Memory between Spaces no longer hides it

Fixed a bug where moving a Memory from one Space to another caused it to disappear from the destination Space's Topics grid. The Memory was still present in the flat "All memories" feed, but the Space page's per-Topic buckets only show Memories whose Topic belongs to that Space — so a Memory carrying its old Space's Topic fell into no bucket. Moving a Memory to a Space now automatically places it in the destination Space's General Topic. Moving to your personal vault clears the Topic assignment entirely, as the personal vault has no Topics.