feat(clients): toolbar sort selector + preserve updated_at on unchanged rows

Frontend
- New Sort dropdown in the clients toolbar covering oldest/newest,
  recently updated, recently online, email A↔Z, most traffic, highest
  remaining, expiring soonest. Default is Oldest first.
- Strip per-column sorter arrows from the Table — all sorting now flows
  through the single dropdown, so the column headers stop competing
  with it.
- Empty state: TeamOutlined icon, t('noData'), text-secondary color
  (matching the inbound/node polish).

Backend
- sortClients: add createdAt, updatedAt and lastOnline cases (with id
  tie-break for stable ordering when timestamps collide).
- Fix Recently updated: SyncInbound was calling tx.Save on every client
  in the inbound, and GORM's autoUpdateTime tag stamped updated_at to
  time.Now() each time — so editing one client bumped ALL of them.
  After the Save, restore each row's preserved updated_at via
  UpdateColumn (skips hooks). The actually-edited client gets its
  fresh stamp from the explicit UpdateColumn at the end of Update().
- Fix periodic updated_at churn: adjustTraffics unconditionally set
  c["updated_at"] = now() for every client in any inbound that had a
  delayed-start expiry, every traffic-stats pass. Turn that into a
  backfill (only when the key is missing), matching the created_at
  treatment one line above.

frontend/src/pages/clients/ClientsPage.css

@@ -178,7 +178,7 @@
 .card-empty {
   text-align: center;
   padding: 40px 16px;
-  opacity: 0.55;
+  color: var(--ant-color-text-secondary);
   display: flex;
   flex-direction: column;
   align-items: center;
@@ -188,5 +188,5 @@
 .clients-empty {
   padding: 32px 0;
   text-align: center;
-  opacity: 0.55;
+  color: var(--ant-color-text-secondary);
 }