refactor(playback): co-locate floating-player-bar + fs-idle-fade hooks (and orphaned playback tests) with their features

src/features/fullscreenPlayer/components/FullscreenPlayerStatic.tsx

@@ -12,7 +12,7 @@ import { useCachedUrl } from '@/ui/CachedImage';
 import { useArtistFanart } from '@/cover/useArtistFanart';
 import { backdropFromConfig } from '@/cover/artistBackdrop';
 import { useThemeStore } from '@/store/themeStore';
-import { useFsIdleFade } from '@/hooks/useFsIdleFade';
+import { useFsIdleFade } from '@/features/fullscreenPlayer/hooks/useFsIdleFade';
 import { useQueueTrackAt } from '@/features/queue';
 import { WaveformSeek } from '@/features/waveform';
 import { FsQueueModal } from '@/features/fullscreenPlayer/components/FsQueueModal';

src/features/fullscreenPlayer/hooks/useFsIdleFade.ts

@@ -0,0 +1,42 @@
+import { useCallback, useEffect, useRef, useState } from 'react';
+
+/** Idle-fade system — flips `isIdle` true after 3 s of no user activity.
+ *  Returns the boolean plus a mousemove handler that's throttled to one reset
+ *  per ~200 ms so cleanup/start timers don't fire on every mouse pixel.
+ *  Also resets on key presses and triggers `onEscape` when the user hits Esc. */
+export function useFsIdleFade(onEscape: () => void) {
+  const [isIdle, setIsIdle] = useState(false);
+  const idleTimer = useRef<ReturnType<typeof setTimeout> | null>(null);
+
+  const resetIdle = useCallback(() => {
+    setIsIdle(false);
+    if (idleTimer.current) clearTimeout(idleTimer.current);
+    idleTimer.current = setTimeout(() => setIsIdle(true), 3000);
+  }, []);
+
+  const lastMoveTime = useRef(0);
+  const handleMouseMove = useCallback(() => {
+    const now = Date.now();
+    if (now - lastMoveTime.current < 200) return;
+    lastMoveTime.current = now;
+    resetIdle();
+  }, [resetIdle]);
+
+  useEffect(() => {
+    // React Compiler set-state-in-effect rule: state set from an external subscription/event callback.
+    // eslint-disable-next-line react-hooks/set-state-in-effect
+    resetIdle();
+    return () => { if (idleTimer.current) clearTimeout(idleTimer.current); };
+  }, [resetIdle]);
+
+  useEffect(() => {
+    const onKey = (e: KeyboardEvent) => {
+      resetIdle();
+      if (e.key === 'Escape') onEscape();
+    };
+    window.addEventListener('keydown', onKey);
+    return () => window.removeEventListener('keydown', onKey);
+  }, [onEscape, resetIdle]);
+
+  return { isIdle, handleMouseMove };
+}