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.

1 files changed, 60 insertions, 0 deletions

diff --git a/src/threaddigest.h b/src/threaddigest.h
new file mode 100644
index 0000000..e78a2ac
--- /dev/null
+++ b/src/threaddigest.h
@@ -0,0 +1,60 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Copyright (C) 2026 Danilo M. <danix@danix.xyz>
+ */
+#ifndef QTMAILDIR_THREADDIGEST_H
+#define QTMAILDIR_THREADDIGEST_H
+
+#include <QList>
+#include <QMetaType>
+#include <QPair>
+#include <QString>
+#include <QVector>
+
+#include "types.h"
+
+/// What the thread dashboard shows, as a plain value.
+///
+/// Crosses the worker boundary like every other structure here: no
+/// notmuch_* pointer, no widget, no model. Everything in it is read from the
+/// INDEX, so building one opens no message files.
+struct ThreadDigest
+{
+    QString threadId;
+
+    /// Display name and message count, most prolific first.
+    QList<QPair<QString, int>> senders;
+
+    /// The unread messages, NEWEST FIRST, at most kUnreadShown of them.
+    ///
+    /// MessageNode rather than MessageRef, because the dashboard DRAWS these
+    /// 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 no-file-opening contract
+    /// above still holds: all four of those are served from the index.
+    QVector<MessageNode> unread;
+
+    /// How many there really are. The list above is capped, so a count taken
+    /// from its size would under-report on exactly the threads that need the
+    /// number most.
+    int unreadTotal = 0;
+
+    int totalCount = 0;
+
+    /// Always kBuckets entries. A fixed count is what keeps the sparkline's
+    /// geometry testable; a thread spanning five days and one spanning two
+    /// years cannot share a bucket size, so the span is what varies and the
+    /// label beneath carries the truth.
+    QVector<int> buckets;
+
+    qint64 firstTimestamp = 0;
+    qint64 lastTimestamp = 0;
+    int busiestBucket = -1;
+
+    static constexpr int kBuckets = 7;
+    static constexpr int kUnreadShown = 5;
+};
+
+Q_DECLARE_METATYPE(ThreadDigest)
+
+#endif // QTMAILDIR_THREADDIGEST_H