Fix TypeScript ref typings and lint errors

src/types/index.ts

@@ -1,4 +1,4 @@
-import type { CSSProperties, RefObject } from 'react'
+import type { CSSProperties, MutableRefObject } from 'react'
 
 export interface SubtitleTrack {
   src: string
@@ -87,8 +87,8 @@ export interface PlayerContextValue {
   videoState: VideoState
   uiState: UIState
   settings: PlayerSettings
-  videoRef: RefObject<HTMLVideoElement>
-  containerRef: RefObject<HTMLDivElement>
+  videoRef: MutableRefObject<HTMLVideoElement | null>
+  containerRef: MutableRefObject<HTMLDivElement | null>
 
   // Video controls
   play: () => void
@@ -109,7 +109,7 @@ export interface PlayerContextValue {
   toggleSettings: () => void
 
   // Settings
-  setQuality: (quality: VideoQuality) => void
+  setQuality: (quality: VideoQuality | null) => void
   setSubtitle: (subtitle: SubtitleTrack | null) => void
   setAudioTrack: (audioTrack: AudioTrack | null) => void
 }