useSubmit holds the idle, sending, done and error state and the request
flow that six forms each wrote out; FormResult shows the success line and
the problem panel; FormStack is the column the link, claude-login,
inference and limits forms each styled by hand. The create-agent, link
account, backend-env and agent-input forms use the submit flow; the link,
claude-login, inference and limits forms use FormStack.
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