Move SSR-stable today loader to data.ts (Modern.js convention)

Inline export const loader from page.tsx didn't run — _ROUTER_DATA
showed loaderData[(lang)/onlineboard/page] = null and useLoaderData()
threw 'Cannot read properties of null'. Modern.js conventional routes
require the loader in a co-located data.ts file.

useLoaderData() now defensively handles null (defaults to undefined,
component falls back to useRef(new Date())). Worst case if loader still
doesn't fire: same hydration drift as before, no crash.

src/routes/[lang]/flights-map/data.ts

@@ -0,0 +1,6 @@
+import { todayYyyymmdd } from "@/shared/utils/datetime/index.js";
+
+/** Modern.js conventional data loader; see /onlineboard/data.ts. */
+export const loader = (): { today: string } => ({
+  today: todayYyyymmdd(),
+});

src/routes/[lang]/flights-map/page.tsx

@@ -15,18 +15,13 @@ import { buildFlightsMapSeo } from "@/features/flights-map/seo.js";
 import { buildFlightsMapJsonLd } from "@/features/flights-map/json-ld.js";
 import { useFeatureFlag } from "@/features/flights-map/hooks/useFeatureFlag.js";
 import { getEnv } from "@/env/index.js";
-import { todayYyyymmdd } from "@/shared/utils/datetime/index.js";
-
 const FlightsMapStartPage = lazy(() =>
   import("@/features/flights-map/components/FlightsMapStartPage.js").then(
     (m) => ({ default: m.FlightsMapStartPage }),
   ),
 );
 
-/** SSR-stable today; see src/routes/[lang]/onlineboard/page.tsx loader. */
-export const loader = (): { today: string } => ({
-  today: todayYyyymmdd(),
-});
+// Loader lives in ./data.ts (Modern.js conventional data file).
 
 export default function FlightsMapPage(): JSX.Element {
   const { t } = useTranslation();
@@ -37,7 +32,8 @@ export default function FlightsMapPage(): JSX.Element {
   // Tile URL read on the server (where process.env is available) and passed
   // down as a prop — the client bundle can't read MAP_TILE_URL itself.
   const tileUrl = env.MAP_TILE_URL;
-  const { today } = useLoaderData() as { today: string };
+  const data = useLoaderData() as { today?: string } | null;
+  const today = data?.today;
   const isEnabled = useFeatureFlag("flightsMap");
 
   if (!isEnabled) {
@@ -55,7 +51,7 @@ export default function FlightsMapPage(): JSX.Element {
     <>
       <SeoHead {...seoProps} jsonLd={jsonLd} />
       <Suspense fallback={<div aria-busy="true">{t("SHARED.LOADING")}</div>}>
-        <FlightsMapStartPage tileUrl={tileUrl} today={today} />
+        <FlightsMapStartPage tileUrl={tileUrl} {...(today ? { today } : {})} />
       </Suspense>
     </>
   );

src/routes/[lang]/onlineboard/data.ts

@@ -0,0 +1,10 @@
+import { todayYyyymmdd } from "@/shared/utils/datetime/index.js";
+
+/**
+ * Modern.js conventional data loader. Co-located with page.tsx.
+ * Computes today on the server, exposes it via useLoaderData() so SSR
+ * and client hydration agree on the same yyyyMMdd.
+ */
+export const loader = (): { today: string } => ({
+  today: todayYyyymmdd(),
+});

src/routes/[lang]/onlineboard/page.tsx

@@ -11,31 +11,23 @@ import { useTranslation } from "@/i18n/provider.js";
 import { SeoHead } from "@/ui/seo/SeoHead.js";
 import { buildOnlineBoardStartSeo } from "@/features/online-board/seo.js";
 import { getEnv } from "@/env/index.js";
-import { todayYyyymmdd } from "@/shared/utils/datetime/index.js";
-
 const OnlineBoardStartPage = lazy(() =>
   import("@/features/online-board/components/OnlineBoardStartPage.js").then(
     (m) => ({ default: m.OnlineBoardStartPage }),
   ),
 );
 
-/**
- * Compute "today" once on the server. The result rides _ROUTER_DATA into
- * the client bundle, so the first client render — the one that hydrates
- * the SSR markup — sees the same yyyyMMdd value the server saw. Without
- * this, components calling `new Date()` during render produce different
- * markup on the two sides and React throws hydration error #423.
- */
-export const loader = (): { today: string } => ({
-  today: todayYyyymmdd(),
-});
+// Loader lives in ./data.ts (Modern.js conventional data file). Result
+// rides _ROUTER_DATA into the client so SSR and hydration agree on the
+// same `today` value, eliminating render-path Date() drift (React #423).
 
 export default function OnlineBoardPage(): JSX.Element {
   const { t } = useTranslation();
   const routeParams = useParams<{ lang: string }>();
   const locale = routeParams.lang ?? "ru-ru";
   const canonicalOrigin = getEnv().PROD_ORIGIN;
-  const { today } = useLoaderData() as { today: string };
+  const data = useLoaderData() as { today?: string } | null;
+  const today = data?.today;
 
   const seoProps = buildOnlineBoardStartSeo(t, locale, canonicalOrigin);
 
@@ -43,7 +35,7 @@ export default function OnlineBoardPage(): JSX.Element {
     <>
       <SeoHead {...seoProps} />
       <Suspense fallback={<div aria-busy="true">{t("SHARED.LOADING")}</div>}>
-        <OnlineBoardStartPage today={today} />
+        <OnlineBoardStartPage {...(today ? { today } : {})} />
       </Suspense>
     </>
   );