feat(ai-studio): the Human task panel follows the design

[jimmeryn]

What

• Title and Description sit in a "General information" section. It is not the SDK's generalInformation, which adds a Status field and needs a node type.
• New "Decider actions" section: Approve is always on, Reject can be turned off (it drops the reject action and its port), and "Reason required" sets reasonRequired. Turning Reject on again restores the default action on the same port, so earlier changes to it are lost. Like the fields section, it is hidden from Run until Reset.
• The panel's hints come in three variants: neutral for guidance, info for how the node behaves, warning for a gap the author has to close. Only the actions hint is a live region.

Open

• The warning background uses --wb-ds-components-snackbar-bg-warning, because there is no semantic --wb-ds-ui-bg-warning-* token yet.
• Turning Reject off leaves the edge from its port, and turning Reject on again brings it back (follow-up: decision-reject-edge-cleanup).
• An emptied required text field shows no error, and nothing says why Approve is disabled.

Not in this PR

• Deadline, rerun and action labels are not authorable yet (follow-up: decision-request-properties-ui).