Fix video/audio desync on Firefox in web video player

Two issues were causing audio to drift out of sync with video on Firefox:

1. fastSeek() seeks video to the nearest keyframe but keeps audio at the
   precise timestamp, causing progressive drift. Disable fastSeek on Firefox
   and use currentTime for precise seeking instead.

2. Low-quality segment flushing only flushed video buffers, leaving stale
   audio data that could desync from newly-loaded video. Flush both audio
   and video buffers together when upgrading quality.

https://claude.ai/code/session_01LGFSY2GgQJ3iw5Cp4XwiuL

src/components/Post/Embed/VideoEmbed/VideoEmbedInner/VideoEmbedInnerWeb.tsx

@@ -169,11 +169,19 @@ function useHLS({
             continue
           }
 
+          // Flush both video and audio buffers to keep them in sync.
+          // Flushing only video leaves stale audio data that can desync,
+          // particularly on Firefox.
           hls.trigger(Hls.Events.BUFFER_FLUSHING, {
             startOffset: lowQualFrag.start,
             endOffset: lowQualFrag.end,
             type: 'video',
           })
+          hls.trigger(Hls.Events.BUFFER_FLUSHING, {
+            startOffset: lowQualFrag.start,
+            endOffset: lowQualFrag.end,
+            type: 'audio',
+          })
 
           flushed.push(lowQualFrag)
         }
@@ -269,6 +277,11 @@ function useHLS({
         endOffset: lowQualFrag.end,
         type: 'video',
       })
+      hls.trigger(Hls.Events.BUFFER_FLUSHING, {
+        startOffset: lowQualFrag.start,
+        endOffset: lowQualFrag.end,
+        type: 'audio',
+      })
       setLowQualityFragments([])
     }
   })

src/components/Post/Embed/VideoEmbed/VideoEmbedInner/web-controls/VideoControls.tsx

@@ -27,7 +27,7 @@ import {Pause_Filled_Corner0_Rounded as PauseIcon} from '#/components/icons/Paus
 import {Play_Filled_Corner0_Rounded as PlayIcon} from '#/components/icons/Play'
 import {Loader} from '#/components/Loader'
 import {Text} from '#/components/Typography'
-import {IS_WEB_MOBILE_IOS, IS_WEB_TOUCH_DEVICE} from '#/env'
+import {IS_WEB_FIREFOX, IS_WEB_MOBILE_IOS, IS_WEB_TOUCH_DEVICE} from '#/env'
 import {GifPresentationControls} from '../../GifPresentationControls'
 import {TimeIndicator} from '../TimeIndicator'
 import {ControlButton} from './ControlButton'
@@ -196,7 +196,9 @@ export function Controls({
   const onSeek = useCallback(
     (time: number) => {
       if (!videoRef.current) return
-      if (videoRef.current.fastSeek) {
+      // Avoid fastSeek on Firefox - it seeks video to the nearest keyframe
+      // but keeps audio at the precise time, causing audio/video desync
+      if (videoRef.current.fastSeek && !IS_WEB_FIREFOX) {
         videoRef.current.fastSeek(time)
       } else {
         videoRef.current.currentTime = time