feat: add the thread dashboard

A widget over a ThreadDigest: header, tags, counts, the unread list capped
with a link to the rest, and an activity sparkline, scrolling under a pinned
action strip. It invents no colours.

ThreadDigest::unread is QVector<MessageNode> rather than QVector<MessageRef>.
The dashboard draws rows for messages it never opens, which is what
MessageNode exists for; MessageRef carries only what the pane needs once a
message is already open and has no subject, sender or date. The struct's
no-file-opening contract still holds, since all four of those fields are
served from the notmuch index.

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