feat(ui): add a Maildir overview under Help

Nothing in the UI reported database-level facts: every query gave a
thread count for that query, and nothing said how much mail there is
overall. A dialog under Help now shows messages, threads and tags from
notmuch, plus the account list from config, since notmuch does not model
accounts at all.

A separate worker call rather than a reuse of requestCounts, which counts
threads to match the row count of a query. This counts messages, which is
what a user means by "how much mail is in here". The test pins 4 messages
in 3 threads against the fixture and fails if they are ever made equal,
so routing both through one count cannot pass unnoticed.

Every field starts at -1 and renders as "unknown" when notmuch could not
answer it. Printing 0 would say the Maildir is empty, and telling someone
their mail is gone is the worst way to report an index that failed to
open.

The dialog opens showing "Counting..." rather than blocking, since
counting every message is not free on a large database. That makes two
lifetimes matter: the reply can arrive after the dialog is closed, so the
label is a QPointer, and the dialog can be closed and reopened while a
count runs, so a generation counter drops the older answer. The test
drains DeferredDelete before firing the late reply, because close()
deletes through deleteLater and without that the dangling case is never
actually reached.

1 files changed, 17 insertions, 0 deletions

diff --git a/src/notmuchworker.h b/src/notmuchworker.h
index 05c1d3a..7d6a587 100644
--- a/src/notmuchworker.h
+++ b/src/notmuchworker.h
@@ -83,6 +83,19 @@ public slots:
     /// refreshing one nobody is looking at is work for nothing.
     void requestCounts(const QStringList &queries, quint64 generation);
 
+    /// Database-level facts for the Maildir overview (item 34): total messages,
+    /// total threads, and the number of tags.
+    ///
+    /// **Messages, not threads**, which is what distinguishes this from
+    /// requestCounts above. That one answers "how many rows will this query
+    /// produce" and counts threads to match the list; this one describes the
+    /// database, where the message total is the number a user means by "how
+    /// much mail is in here".
+    ///
+    /// Counting every message is not free on a large database, so this is
+    /// called when the dialog is opened and never on a timer.
+    void requestDatabaseStats(quint64 generation);
+
 signals:
     void threadsReady(const QVector<ThreadSummary> &threads, quint64 generation);
     void queryFinished(int totalThreads, quint64 generation);
@@ -95,6 +108,10 @@ signals:
     /// positional correspondence the caller relies on always holds.
     void countsReady(const QVector<int> &counts, quint64 generation);
 
+    /// Fields left at -1 are ones notmuch could not answer, which the dialog
+    /// renders as unknown rather than as zero.
+    void databaseStatsReady(const DatabaseStats &stats, quint64 generation);
+
     void errorOccurred(const QString &message);
 
 private: