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/lib/storage.ts

@@ -50,8 +50,9 @@ export const LS = {
   plexQ: "siftlode.plexQ",
   plexPlaylistLayout: "siftlode.plexPlaylistLayout",
   plexPlayerPrefs: "siftlode.plexPlayerPrefs",
-  // The YouTube modal player's volume/mute. A YT.Player instance is created per video and starts
-  // at full volume, so without this every next video is loud again (same idea as plexPlayerPrefs).
+  // The YouTube modal player's volume (`{ volume: 0–100 }`; 0 IS muted — there is no separate
+  // flag). A YT.Player instance is created per video and starts at full volume, so without this
+  // every next video is loud again (same idea as plexPlayerPrefs).
   ytPlayerPrefs: "siftlode.ytPlayerPrefs",
   notifHistory: "siftlode.notifications",
   notifSettings: "siftlode.notifSettings",