Keep DayTabs enabled until calendar API resolves

With empty availableDates every day button rendered as [disabled],
leaving the user unable to navigate between days while the /days API
loads (or for routes where the calendar endpoint hasn't been wired
yet). Treat an empty availableDates array as 'unknown' — don't disable
anything, matching Angular's behaviour where tabs are tappable until
the upstream tells us a specific day has no flights.

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

@@ -39,7 +39,12 @@ export const DayTabs: FC<DayTabsProps> = ({
 
   const [currentPage, setCurrentPage] = useState(initialPage);
 
+  // Empty availableDates means the calendar API hasn't resolved yet (or
+  // the route has no day data). Treat every date as tappable in that
+  // case — matches Angular where the tabs stay enabled until we *know*
+  // the upstream reports no flights for a given day.
   const availableSet = useMemo(() => new Set(availableDates), [availableDates]);
+  const disableByAvailability = availableDates.length > 0;
 
   const visibleDates = allDates.slice(
     currentPage * PAGE_SIZE,
@@ -68,7 +73,7 @@ export const DayTabs: FC<DayTabsProps> = ({
               key={date}
               date={date}
               isActive={date === selectedDate}
-              isDisabled={!availableSet.has(date)}
+              isDisabled={disableByAvailability && !availableSet.has(date)}
               locale={locale}
               onClick={onNavigate}
             />