preserve video's last known time when scrolling away (#6239)

src/view/com/util/post-embeds/VideoEmbed.web.tsx

···
     useActiveVideoWeb()
   const [onScreen, setOnScreen] = useState(false)
   const [isFullscreen] = useFullscreen()
+  const lastKnownTime = useRef<number | undefined>()
 
   useEffect(() => {
     if (!ref.current) return
···
               active={active}
               setActive={setActive}
               onScreen={onScreen}
+              lastKnownTime={lastKnownTime}
             />
           </ViewportObserver>
         </ErrorBoundary>

src/view/com/util/post-embeds/VideoEmbedInner/VideoEmbedInnerWeb.tsx

···
   active,
   setActive,
   onScreen,
+  lastKnownTime,
 }: {
   embed: AppBskyEmbedVideo.View
   active: boolean
   setActive: () => void
   onScreen: boolean
+  lastKnownTime: React.MutableRefObject<number | undefined>
 }) {
   const containerRef = useRef<HTMLDivElement>(null)
   const videoRef = useRef<HTMLVideoElement>(null)
···
     setHlsLoading,
   })
 
+  useEffect(() => {
+    if (lastKnownTime.current && videoRef.current) {
+      videoRef.current.currentTime = lastKnownTime.current
+    }
+  }, [lastKnownTime])
+
   return (
     <View style={[a.flex_1, a.rounded_md, a.overflow_hidden]}>
       <div ref={containerRef} style={{height: '100%', width: '100%'}}>
···
             preload="none"
             muted={!focused}
             aria-labelledby={embed.alt ? figId : undefined}
+            onTimeUpdate={e => {
+              lastKnownTime.current = e.currentTarget.currentTime
+            }}
           />
           {embed.alt && (
             <figcaption