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, 16 insertions, 0 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 25bd39d..851e318 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -11,6 +11,22 @@ point at which they are stable.
 
 ## [Unreleased]
 
+### Added
+
+- **A sync now fetches only the accounts you have edited.** Tagging mail in one
+  account and syncing no longer pulls every other account as well. A sync with
+  nothing outstanding is a plain fetch and still covers everything, since
+  narrowing that to wherever the last edit happened to be would quietly stop
+  collecting mail everywhere else.
+- **An optional `channel` key per account**, naming the mbsync channel when it
+  differs from the section key. It defaults to the key, so accounts whose two
+  names already agree need no change. The two can genuinely diverge: a QSettings
+  section key may carry dots that the channel does not, and mbsync treats an
+  unknown channel as fatal rather than skipping it.
+- **`assets/mailsync.sh` takes channel names as arguments**, syncing all
+  channels when given none. A replacement sync script that ignores its arguments
+  still works, it just always syncs everything.
+
 ## [0.11.0] - 2026-08-07
 
 The empty message pane now carries the application's own identity and the