OnlineBoardFilter: validate departure===arrival mismatch with inline error (Angular parity)

src/features/online-board/components/OnlineBoardFilter.tsx

@@ -131,6 +131,10 @@ export const OnlineBoardFilter: FC<OnlineBoardFilterProps> = ({
     initialTab === "flight" && initialDate ? yyyymmddToDate(initialDate) : null,
   );
   const [flightNumberError, setFlightNumberError] = useState<string | null>(null);
+  // Mirrors Angular OnlineBoardRouteFilterValidationService state: a
+  // single rejection message shown next to the arrival input when
+  // departure === arrival. Cleared as soon as the user edits either field.
+  const [routeSameCitiesError, setRouteSameCitiesError] = useState<string | null>(null);
 
   // Route fields
   const [routeDepartureCode, setRouteDepartureCode] = useState<string>(initialDeparture ?? "");
@@ -250,6 +254,15 @@ export const OnlineBoardFilter: FC<OnlineBoardFilterProps> = ({
       const arrCode = routeArrivalCode.trim().toUpperCase();
       if (!depCode && !arrCode) return;
 
+      // Angular's validation rejects identical departure/arrival codes.
+      // Surface the same translated message and bail out before building
+      // a URL that would return an empty board.
+      if (depCode && arrCode && depCode === arrCode) {
+        setRouteSameCitiesError("SHARED.ARRIVAL-EQUALS-DEPARTURE-ERROR");
+        return;
+      }
+      setRouteSameCitiesError(null);
+
       // Only send time range when the user has narrowed the slider, so
       // default 00:00–24:00 searches keep the URL clean (Angular spreads
       // `...this.timeRange` which is undefined until interaction).
@@ -411,7 +424,10 @@ export const OnlineBoardFilter: FC<OnlineBoardFilterProps> = ({
                           : t("SHARED.CITY_PLACEHOLDER")
                       }
                       value={routeDepartureCode}
-                      onChange={setRouteDepartureCode}
+                      onChange={(code) => {
+                        setRouteDepartureCode(code);
+                        if (routeSameCitiesError) setRouteSameCitiesError(null);
+                      }}
                       dictionaries={dictionaries}
                       testIdPrefix="route-departure"
                     />
@@ -437,10 +453,21 @@ export const OnlineBoardFilter: FC<OnlineBoardFilterProps> = ({
                           : t("SHARED.CITY_PLACEHOLDER")
                       }
                       value={routeArrivalCode}
-                      onChange={setRouteArrivalCode}
+                      onChange={(code) => {
+                        setRouteArrivalCode(code);
+                        if (routeSameCitiesError) setRouteSameCitiesError(null);
+                      }}
                       dictionaries={dictionaries}
                       testIdPrefix="route-arrival"
                     />
+                    {routeSameCitiesError && (
+                      <div
+                        className="validation-tooltip"
+                        data-testid="route-same-cities-error"
+                      >
+                        {t(routeSameCitiesError)}
+                      </div>
+                    )}
 
                     <div className="calendar">
                       <label className="label--filter">