[allison-truhlar]

What changed

• The Layer Cart tracks a channel by its position in the dataset instead
  of its name. Checking one Gpb5 round no longer checks every Gpb5.
• Channels that share a label get unique Neuroglancer layer names
  (Gpb5, Gpb5 (2), Gpb5 (3)), so every selected round loads.
  Checkout also renames layers that collide across datasets, such as two
  datasets that each have DAPI.
• Long channel lists in the Layer Cart and the Sources tab wrap into
  columns when the panel is wide enough.
• The Sources tab lists every channel of a dataset added channel by
  channel, with the channels in the View checked. Checking an unlisted
  channel adds it as a shown layer.

Why

A dataset with repeated probe rounds showed one set of channels checked
in the cart and loaded a different set in Neuroglancer. Adding another
channel from a dataset already in the View also meant a trip back to the
Add data tab.

Testing

• Frontend unit tests pass (pixi run test-frontend), including new
  tests for repeated channel names, cross-dataset layer names, the full
  channel list in Sources, and the "Added 1 channel" toast.
• ESLint is clean. pixi run node-check reports only the 6 errors
  already on main.

@krokicki
Clickup id: 17tkuw5z7va