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, 13 insertions, 0 deletions

diff --git a/src/config.h b/src/config.h
index 84f02df..d3ee767 100644
--- a/src/config.h
+++ b/src/config.h
@@ -45,6 +45,19 @@ struct Account
     /// bit of information. This renames nothing in notmuch, only the display.
     QString label;
 
+    /// mbsync channel name, when it differs from the key. Optional, and empty
+    /// for most accounts: see syncChannel().
+    QString channel;
+
+    /// The mbsync channel to sync this account, for item 49's per-account sync.
+    ///
+    /// Defaults to the key, which is right for most accounts, but the two are
+    /// genuinely separate names and cannot be collapsed. A QSettings section
+    /// key may carry dots that the channel does not ([account.mail-first.last]
+    /// against the channel `mail-firstlast`), and mbsync exits nonzero on a
+    /// channel it does not know, which qtmaildir would report as a failed sync.
+    QString syncChannel() const { return channel.isEmpty() ? key : channel; }
+
     bool isValid() const { return !key.isEmpty() && !maildir.isEmpty(); }
 
     /// Restricts a notmuch query to this account's subtree.