feat(casting): add cast autoplay countdown atom

utils/atoms/castAutoplay.ts

@@ -0,0 +1,17 @@
+/**
+ * Countdown state for Chromecast next-episode autoplay. The watcher
+ * (`useCastAutoplay`) writes it; the casting-player overlay reads it.
+ */
+
+import type { BaseItemDto } from "@jellyfin/sdk/lib/generated-client/models";
+import { atom } from "jotai";
+
+export interface CastAutoplayState {
+  /** The episode queued to play next. */
+  nextEpisode: BaseItemDto;
+  /** Seconds left before it loads. */
+  secondsRemaining: number;
+}
+
+/** Active cast autoplay countdown, or null when none is running. */
+export const castAutoplayAtom = atom<CastAutoplayState | null>(null);