Mpi H3 Length

Convert a wanted duration into a valid MiniMax H3 frame count. H3 only generates n % 17 == 5 frames at 24 fps, so exact whole seconds are mostly impossible - 8 s is the shortest one that lands. Wire `frames` into the H3 node's `length`, and show `seconds` in the UI instead of what was asked for. `in_trained_range` is false outside 124-362 frames (~5.2-15.1 s), where the model still runs but was not trained.