feat(sync): sync only the accounts with unsynced edits

A sync ran mbsync -a regardless of what changed, so tagging mail in one
account fetched all of them. The account set was not a parameter anywhere
on the path: MailSync::start() took no arguments and the script hardcoded
-a, so nothing between a tag edit and mbsync carried which account changed.

Track which accounts have edits and pass their mbsync channels through to
the script, which now takes channel names and falls back to -a when given
none. An empty set means all accounts, per the request: a sync with nothing
pending is a fetch, and narrowing that to wherever the last edit landed
would quietly stop collecting mail everywhere else.

The channel is a new optional per-account key rather than the section key.
The two names genuinely diverge, because a QSettings section key may carry
dots that the channel does not, and mbsync treats an unknown channel as
fatal rather than skipping it, so key-as-channel would fail those accounts'
syncs outright rather than degrade. It defaults to the key, so accounts
whose two names already agree need no config change.

The edited-account set is deliberately not netted the way the pending-edit
map is: that map tracks the index, where a tag removed and re-added leaves
nothing outstanding, while this tracks the mail store, where both writes
have already renamed files that mbsync still has to propagate. It is also
snapshotted before flushHeldEdits(), which inserts into it synchronously
rather than on a queued reply, so a successful sync cannot clear accounts
whose edits it never carried.

Closes item 49.

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

1 files changed, 43 insertions, 0 deletions

diff --git a/tests/test_threadlistmodel.cpp b/tests/test_threadlistmodel.cpp
index 82686e5..f84bbab 100644
--- a/tests/test_threadlistmodel.cpp
+++ b/tests/test_threadlistmodel.cpp
@@ -28,6 +28,9 @@ class TestThreadListModel : public QObject
     Q_OBJECT
 private slots:
     void startsEmpty();
+    void accountKeysComeFromTheAccountTags();
+    void accountKeysCoverAThreadSpanningTwoAccounts();
+    void accountKeysAreEmptyForAnUnknownThread();
     void appendsBatches();
     void appendingEmptyBatchIsNoOp();
     void clearResetsModel();
@@ -73,6 +76,46 @@ static ThreadSummary makeThread(const QString &id, const QString &subject)
     return t;
 }
 
+void TestThreadListModel::accountKeysComeFromTheAccountTags()
+{
+    // Item 49 reads this to decide which mbsync channels a sync needs. Only
+    // account tags count: a functional tag names no mailbox.
+    ThreadListModel model;
+    ThreadSummary t = makeThread(QStringLiteral("t1"), QStringLiteral("Hi"));
+    t.tags.append(TagColors::tagForAccountKey(QStringLiteral("work")));
+    model.appendBatch({ t });
+
+    QCOMPARE(model.accountKeysForThread(QStringLiteral("t1")),
+             QStringList{ QStringLiteral("work") });
+}
+
+void TestThreadListModel::accountKeysCoverAThreadSpanningTwoAccounts()
+{
+    // The row shows one chip, but tagging this thread touches files under both
+    // mailboxes. Returning only the first would strand the other's edits.
+    ThreadListModel model;
+    ThreadSummary t = makeThread(QStringLiteral("t1"), QStringLiteral("Hi"));
+    t.tags.append(TagColors::tagForAccountKey(QStringLiteral("work")));
+    t.tags.append(TagColors::tagForAccountKey(QStringLiteral("personal")));
+    model.appendBatch({ t });
+
+    QStringList keys = model.accountKeysForThread(QStringLiteral("t1"));
+    keys.sort();
+    QCOMPARE(keys, (QStringList{ QStringLiteral("personal"),
+                                 QStringLiteral("work") }));
+}
+
+void TestThreadListModel::accountKeysAreEmptyForAnUnknownThread()
+{
+    // A thread the model no longer holds must yield nothing rather than
+    // matching some other row.
+    ThreadListModel model;
+    model.appendBatch({ makeThread(QStringLiteral("t1"),
+                                   QStringLiteral("Hi")) });
+
+    QVERIFY(model.accountKeysForThread(QStringLiteral("nope")).isEmpty());
+}
+
 void TestThreadListModel::startsEmpty()
 {
     ThreadListModel model;