fix(r5): address the fourth /code-review high round — 6 findings

- Feed memoizes onClose (like Playlists already did): the inline closure re-ran
  PlayerModal's keydown effect on every Feed render, which since the debounce
  landed also flushed the volume write and re-stole focus
- the player reads its volume with readAccountMerged again — a partial stored
  object made the level undefined -> NaN -> a persisted null; applyVolume also
  refuses a non-finite level outright
- that read moved into a lazy useState initializer (was running per render)
- the two RSS except branches share one tail, so a counter can't drift again
- run_ffmpeg tests cover the SIGTERM->SIGKILL escalation and the lingering-
  process path; dropped an assertion that only restated the fixture
- storage.ts documents the actual { volume } shape (0 IS muted)

frontend/src/components/Feed.tsx

@@ -137,6 +137,11 @@ export default function Feed({
     (video: Video, startAt: number | null = null) => setActiveVideo({ video, startAt }),
     [],
   );
+  // Stable identity, like Playlists' `closePlayer`: PlayerModal's keydown/scroll-lock effect
+  // depends on `onClose`, so a fresh closure every Feed render re-subscribed those listeners,
+  // re-ran focusModal() under the open player, dropped its in-flight timers and flushed the
+  // debounced volume write on each pass.
+  const closePlayer = useCallback(() => setActiveVideo(null), []);
 
   const ytActive = !!ytSearch;
   // How many live-search results to gather (the count selector replaces a manual "load more";
@@ -484,7 +489,7 @@ export default function Feed({
               video={activeVideo.video}
               startAt={activeVideo.startAt}
               queue={ytPlayerQueue}
-              onClose={() => setActiveVideo(null)}
+              onClose={closePlayer}
               onState={onState}
             />
           </Suspense>
@@ -739,7 +744,7 @@ export default function Feed({
             video={activeVideo.video}
             startAt={activeVideo.startAt}
             queue={playerQueue}
-            onClose={() => setActiveVideo(null)}
+            onClose={closePlayer}
             onState={onState}
           />
         </Suspense>