fix(e10): review round 3 — settle the focus race in a browser instead of guessing again

Round 3 reviewed round 2's diff. Trend across the loop: 32 findings over
the whole epic, 15 over round 1's fixes, 11 over round 2's. What it caught
is mostly the same shape as before — a guard narrowed until it was wrong
in the other direction — plus one item this branch had now got wrong
three different ways.

Backspace ownership over the player, third and final answer. Round 1
deferred to nothing (the cheat sheet took the key and the player stopped
underneath it). Round 1's fix deferred to `anyLayerAbove` (the player's
own gear/tracks popovers push layers too, so it stopped closing them).
Round 2's fix named the sheet and palette (the app-wide ErrorDialog is
neither, so the player tore down under it). The rule that actually holds
is not a list of overlays: the player's own panels answer first, because
handleBack's cascade is what closes them, and anything else layered above
takes the key.

The rail's inline rename is settled by measurement rather than argument.
Two rounds reasoned about whether a React passive effect lands before or
after floating-ui's return-focus microtask and reached opposite
conclusions; a jsdom harness built to decide it reproduced no failure for
ANY shape, because focus never enters the menu there — so it was deleted
rather than kept as a test that discriminates nothing. Checked in a real
browser instead: the field opens and holds focus. The shipped fix works,
and the comment now says what was measured rather than what was inferred.

The same check found that round 1's OTHER rail fix never worked at all.
Escape does run its own handler (typing then Escape leaves the name
unchanged, so it cancels rather than blur-commits) and the row button is
focusable when focused from outside — yet focus still lands on <body>.
Three explanations, none of which survived. So the refocus machinery is
reverted to the shape that shipped before this epic and the gap is
documented where the next person will hit it, with the evidence and an
instruction to instrument before theorising. A fourth guess would have
been worse than a known bug.

Also: the two comments in Playlists.tsx that still claimed the disabled
menu rows explain themselves — untrue since round 2 reverted `inert`, and
exactly the comment-vs-code drift CLAUDE.md calls the source of the worst
bugs here. And the move-failure toast printed HttpError.detail verbatim,
which for the reachable 404 is the backend's bare English "Unknown
playlist"; only a structured detail has been through localizeDetail, so
only a structured detail reaches the toast now.

frontend/src/components/PlexPlayer.tsx

@@ -710,14 +710,22 @@ export default function PlexPlayer({ itemId, onClose, queue }: Props) {
         stats: () => setStatsOpen((v) => !v),
         back: (ev) => {
           // HTPC-style "stop & back to the feed" — mirrors the mouse Back button (shared cascade).
-          // Opening the cheat sheet (or the palette) over the player and pressing ⌫ used to stop
-          // playback and drop to the Plex grid while the overlay stayed up: Modal stopPropagation()s
-          // Escape ONLY, and its focused close button is not a typing target, so the key sailed
-          // through (E10 review round 1). The first fix deferred to `anyLayerAbove`, which was far
-          // too broad — the player's OWN gear/tracks menus register layers via useDismiss, so ⌫
-          // stopped closing them and did nothing at all (round 2). Only these two app-wide overlays
-          // own Backspace, so name them rather than every layer.
-          if (helpOpenRef.current || paletteOpenRef.current) return;
+          // Who owns ⌫ when something is open over the player. Modal stopPropagation()s Escape ONLY
+          // and its focused close button is not a typing target, so the key reaches us regardless.
+          // Two wrong answers so far: deferring to nothing at all (round 1's bug — the cheat sheet
+          // took ⌫ and the player stopped underneath it), then deferring to `anyLayerAbove` outright
+          // (round 1's fix, round 2's bug — the player's OWN gear/tracks popovers push layers too, so
+          // ⌫ stopped closing them), then naming just the sheet and palette (round 2's fix, round 3's
+          // bug — the app-wide ErrorDialog is neither, so ⌫ tore the player down under it).
+          // The rule that holds: the player's own panels answer first, because `handleBack`'s cascade
+          // is what closes them; anything else layered above us takes the key.
+          const ownPanelOpen =
+            menuOpenRef.current ||
+            infoOpenRef.current ||
+            statsOpenRef.current ||
+            upNextOpenRef.current;
+          if (!ownPanelOpen && layerIdRef.current != null && anyLayerAbove(layerIdRef.current))
+            return;
           ev.preventDefault();
           if (skipProgressRef.current != null)
             cancelAutoSkipRef.current(); // cancel auto-skip, don't go back