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, 10 insertions, 0 deletions

diff --git a/src/threadlistmodel.h b/src/threadlistmodel.h
index 461e467..2eaa88e 100644
--- a/src/threadlistmodel.h
+++ b/src/threadlistmodel.h
@@ -120,6 +120,16 @@ public:
 
     ThreadSummary threadAt(int row) const;
 
+    /// The account keys behind a thread's account tags, for item 49's
+    /// per-account sync.
+    ///
+    /// Returns every one of them, not the first: the thread list shows only one
+    /// chip per row, but a thread whose messages landed in two mailboxes really
+    /// does span two accounts, and tagging it touches files under both. Syncing
+    /// only the one that happens to be shown would strand the other's edits.
+    /// Empty when the thread is unknown or carries no account tag.
+    QStringList accountKeysForThread(const QString &threadId) const;
+
     /// Applies a tag change locally so the UI updates before the worker
     /// confirms. To revert a failed write, call again with added and removed
     /// swapped.