feat(types): add MessageNode for message rows in the thread list

A message row has to be drawn without opening the message, so it needs sender,
subject and date. MessageRef carries none of them: it exists for rendering a
thread into the pane and holds only id, path, tags and matched.

depth defaults to 0, the thread's first message, which the root row stands for
rather than a child row. threadId is carried so a batch of nodes names the
thread it belongs to without the caller tracking it alongside.

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