fix(dashboard): review fixes — error toasts, dedupe socket, POS guards

- Global MutationCache.onError safety net so mutations without their own onError
  no longer fail silently (skips ones that handle errors → no double toast).
- Notifications feed no longer opens its own SignalR connection; it reuses the
  one in useOrderAlerts (was double sockets + double cache churn per session).
- "Send test notification" now works on the settings page (force flag bypasses
  the tab-visible guard) instead of silently doing nothing.
- POS: re-entry guard on payment confirm (no duplicate payment on double-tap);
  notes on already-sent lines are read-only (a note-only edit was silently lost);
  ORDER_ALREADY_CLOSED surfaced with a clear Persian message.
- Reservation Confirm/Cancel/Complete buttons disabled while pending.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

web/dashboard/src/components/reservations/reservations-screen.tsx

@@ -234,6 +234,7 @@ export function ReservationsScreen() {
                         <Can permission="EditReservation">
                           <Button
                             size="sm"
+                            disabled={updateStatus.isPending}
                             onClick={() => updateStatus.mutate({ id: r.id, status: "Confirmed" })}
                           >
                             {t("confirm")}
@@ -243,6 +244,7 @@ export function ReservationsScreen() {
                           <Button
                             size="sm"
                             variant="outline"
+                            disabled={updateStatus.isPending}
                             onClick={() => updateStatus.mutate({ id: r.id, status: "Cancelled" })}
                           >
                             {t("cancel")}
@@ -260,6 +262,7 @@ export function ReservationsScreen() {
                         <Button
                           size="sm"
                           variant="outline"
+                          disabled={updateStatus.isPending}
                           onClick={() => updateStatus.mutate({ id: r.id, status: "Completed" })}
                         >
                           {t("markCompleted")}