perf(frontend): fix resource leaks and disable deep reactivity on media index

- useMediaWebSocket: shallowRef mediaIndex/tasks/loading/error/connected
  to eliminate Proxy overhead on the entire library data structure.
- useMediaWebSocket: replace per-task setTimeout leak with single 3s
  sweep interval for completed task cleanup.
- App.vue: remove {deep:true} watcher on mediaIndex; shallow ref change
  is sufficient to trigger search worker sync.
- useGamepadNavigation: rAF only when gamepads are connected; idle
  fallback to 500ms setTimeout to stop permanent 60fps CPU drain.
- useKeyboardNavigation: deduplicate synced scroll rAF requests to
  prevent overlapping animation frames.
- MediaDetail/SeriesFullView/CollageHero: pause, clear src, and load()
  video elements on unmount and before ref replacement to release
  decoder/memory resources.
- MediaDetail/SeriesFullView: clear all volume fade intervals on
  unmount to stop interval timer leaks.

frontend/src/components/SeriesFullView.vue

@@ -468,6 +468,13 @@ const videoRefs = ref<Map<string, HTMLVideoElement>>(new Map())
 let videoIndex = 0
 const safariAutoplay = isSafariBrowser()
 
+function cleanupVideo(video: HTMLVideoElement | null | undefined) {
+  if (!video) return
+  video.pause()
+  video.src = ""
+  video.load()
+}
+
 // Set video ref with staggered playback
 function setVideoRef(el: HTMLVideoElement | null, key: string) {
   if (el) {
@@ -484,6 +491,10 @@ function setVideoRef(el: HTMLVideoElement | null, key: string) {
       safariAutoplay ? 0 : index * 200,
     )
   } else {
+    const old = videoRefs.value.get(key)
+    if (old) {
+      cleanupVideo(old)
+    }
     videoRefs.value.delete(key)
   }
 }
@@ -649,6 +660,16 @@ onMounted(() => {
 
 onUnmounted(() => {
   window.removeEventListener("mediahive:gamepad-action", handleGamepadAction as EventListener)
+  // Clear all volume fade intervals
+  for (const interval of volumeFadeIntervals.values()) {
+    clearInterval(interval)
+  }
+  volumeFadeIntervals.clear()
+  // Pause and unload all video elements
+  for (const video of videoRefs.value.values()) {
+    cleanupVideo(video)
+  }
+  videoRefs.value.clear()
 })
 </script>