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, 20 insertions, 0 deletions

diff --git a/src/mainwindow.h b/src/mainwindow.h
index 579597c..2d41b5e 100644
--- a/src/mainwindow.h
+++ b/src/mainwindow.h
@@ -19,6 +19,7 @@
 #pragma once
 
 #include <QHash>
+#include <QSet>
 #include <QMainWindow>
 #include <QPointer>
 #include <QThread>
@@ -524,6 +525,25 @@ private:
     /// the UI thread one user action at a time.
     TagChange m_pendingChange;
     QStringList m_pendingThreadIds;
+
+    /// Account keys whose mail store has edits a sync has not yet carried,
+    /// for item 49's per-account sync.
+    ///
+    /// Deliberately NOT netted the way m_pendingTagEdits is. That map tracks
+    /// the INDEX, where removing a tag and re-adding it leaves nothing
+    /// outstanding; this tracks the MAIL STORE, where both writes have already
+    /// renamed files that mbsync still has to propagate. Netting this to empty
+    /// would skip the very account whose files changed.
+    ///
+    /// Populated where the threads are known, since TagChange carries message
+    /// ids and the account is a property of the thread. Cleared only by a
+    /// SUCCESSFUL sync, alongside the pending-edit map.
+    QSet<QString> m_editedAccounts;
+
+    /// The channel names for m_editedAccounts, resolved through the config.
+    /// Empty means sync everything, which is what a fetch with nothing pending
+    /// has to do.
+    QStringList pendingSyncChannels() const;
 };
 
 /// Undo entry for a tag change over a set of threads.