Translate remaining route-level English strings

- Flights-map empty/error states: 'Failed to load routes. Please try
  again.' and 'No directions found.' now use existing translation keys
  (BOARD.LOAD-FAILED + FLIGHTS-MAP.NO_DIRECTIONS_INFO).
- Flights-map feature-flag fallback '404 - Page Not Found / This
  feature is currently unavailable.' reduced to the translated
  PAGE404.HEADER string.
- Suspense fallbacks on /onlineboard, /schedule, /flights-map and
  /popular route pages now render the new SHARED.LOADING ('Загрузка…')
  instead of hardcoded 'Loading...'.

src/features/flights-map/components/FlightsMapStartPage.tsx

@@ -355,7 +355,7 @@ export const FlightsMapStartPage: FC = () => {
                 role="alert"
                 data-testid="map-error"
               >
-                Failed to load routes. Please try again.
+                {t("BOARD.LOAD-FAILED")}
               </div>
             )}
 
@@ -364,7 +364,7 @@ export const FlightsMapStartPage: FC = () => {
                   className="flights-map-start__empty"
                   data-testid="map-no-directions"
                 >
-                  No directions found.
+                  {t("FLIGHTS-MAP.NO_DIRECTIONS_INFO")}
                 </div>
               )}
           </div>