feat(rules): every Maildir folder in the Folder dropdown

The dropdown was built from config, which names one subtree per account
and nothing below it, so it offered five entries and no way to say
Drafts or Sent. A rule wants to target those as often as a whole
account.

NotmuchWorker gains requestFolders/foldersReady, walking the tree from
notmuch_database_get_path() and listing every directory holding cur/.
It belongs there because the database root is notmuch's database.path
and the worker owns the only handle that can answer for it; putting the
root in config would be the second source of truth the design refuses.

From the disk rather than from the index: a folder mbsync created and
nothing has landed in yet is still a folder a rule may target, and a
list derived from indexed message paths would not offer it.

The two tests build their own fixture rather than extending the shared
one, which needs a nested folder and would otherwise move seven count
assertions in unrelated tests. Mutation-checked: flattening the walk to
non-recursive fails the listing test.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

1 files changed, 37 insertions, 0 deletions

diff --git a/src/notmuchworker.cpp b/src/notmuchworker.cpp
index 94fb79c..6aae397 100644
--- a/src/notmuchworker.cpp
+++ b/src/notmuchworker.cpp
@@ -20,6 +20,9 @@
 
 #include <notmuch.h>
 
+#include <QDir>
+#include <QDirIterator>
+#include <QFileInfo>
 #include <QSet>
 
 #include <cstdlib>
@@ -678,3 +681,37 @@ void NotmuchWorker::requestMessageCounts(const QStringList &queries,
 
     emit messageCountsReady(counts, generation);
 }
+
+void NotmuchWorker::requestFolders()
+{
+    if (!openReadOnly())
+        return;
+
+    const QString root = QString::fromUtf8(notmuch_database_get_path(m_db));
+    if (root.isEmpty()) {
+        emit errorOccurred(
+            QStringLiteral("notmuch reports no database path."));
+        return;
+    }
+
+    // A Maildir folder is a directory holding cur/. Testing for that rather
+    // than listing every directory keeps the plumbing (cur, new, tmp) and an
+    // account's container directory out of the list; neither is somewhere mail
+    // is filed. Hidden directories are skipped, which is what excludes
+    // .notmuch itself.
+    QStringList folders;
+    QDirIterator it(root, QDir::Dirs | QDir::NoDotAndDotDot,
+                    QDirIterator::Subdirectories);
+    const QDir rootDir(root);
+    while (it.hasNext()) {
+        const QString path = it.next();
+        if (!QFileInfo::exists(path + QStringLiteral("/cur")))
+            continue;
+        folders.append(rootDir.relativeFilePath(path));
+    }
+
+    // Sorted, so the dropdown keeps one order across openings. QDirIterator
+    // walks in filesystem order, which is neither stable nor alphabetical.
+    folders.sort();
+    emit foldersReady(folders);
+}