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.

web/translation/en-US.json

@@ -38,7 +38,7 @@
   "edit": "Edit",
   "delete": "Delete",
   "reset": "Reset",
-  "noData": "No data.",
+  "noData": "Nothing here yet",
   "copySuccess": "Copied successfully",
   "sure": "Sure",
   "encryption": "Encryption",
@@ -461,6 +461,15 @@
       "searchPlaceholder": "Search email, comment, sub ID, UUID, password, auth…",
       "filterTitle": "Filter clients",
       "clearAllFilters": "Clear all",
+      "sortOldest": "Oldest first",
+      "sortNewest": "Newest first",
+      "sortRecentlyUpdated": "Recently updated",
+      "sortRecentlyOnline": "Recently online",
+      "sortEmailAZ": "Email A→Z",
+      "sortEmailZA": "Email Z→A",
+      "sortMostTraffic": "Most traffic",
+      "sortHighestRemaining": "Highest remaining",
+      "sortExpiringSoonest": "Expiring soonest",
       "has": "Has",
       "hasNot": "Doesn't have",
       "title": "Clients",
@@ -496,7 +505,6 @@
       "resetAllTraffics": "Reset all client traffic",
       "resetAllTrafficsTitle": "Reset all client traffic?",
       "resetAllTrafficsContent": "Every client's up/down counter drops to zero. Quotas and expiry are not affected. This cannot be undone.",
-      "empty": "No clients yet — add one to get started.",
       "deleteConfirmTitle": "Delete client {email}?",
       "deleteConfirmContent": "This removes the client from every attached inbound and drops its traffic record. This cannot be undone.",
       "deleteSelected": "Delete ({count})",