[allison-truhlar]

Stacked on #475.

What changed

• An "All channels" box in the Layer Cart selects or clears every channel of a dataset, and shows a partial state when only some are checked.
• Checkout generates each dataset's state once instead of once per selected channel. Before this change, a dataset with every channel selected re-read its metadata for each channel.

Testing

• pixi run test-frontend passes.

@krokicki