feat(plex): player HTPC polish — Stop button, keyboard help, time-remaining + clock, scroll-restore

- Explicit Stop button (stops playback + returns to the library).
- Keyboard cheat-sheet overlay toggled with 'H' (movie hides the episode-nav row).
- Backspace = stop & back to feed (HTPC-remote convention); closes the help first.
- Time-remaining readout beside the seek bar + a live wall clock in the top bar
  (both fade with the controls).
- PlexBrowse restores the grid scroll position when returning from the player, so
  Back lands on the same card instead of the top of the library.
- New plex.player i18n keys (stop/help/keys.*) in en/hu/de.

frontend/src/components/PlexBrowse.tsx

@@ -1,4 +1,4 @@
-import { lazy, Suspense, useEffect, useRef, type CSSProperties } from "react";
+import { lazy, Suspense, useEffect, useLayoutEffect, useRef, type CSSProperties } from "react";
 import { useTranslation } from "react-i18next";
 import { useInfiniteQuery, useQuery, useQueryClient } from "@tanstack/react-query";
 import { ArrowLeft, CheckCircle2, Play } from "lucide-react";
@@ -34,6 +34,19 @@ export default function PlexBrowse({ q, library, show, sort }: Props) {
   const dq = useDebounced(q.trim(), 350);
   const sub = useHistorySubview<Sub>({ kind: "grid" });
 
+  // Opening a show/player replaces the grid entirely (the component returns early below), so the
+  // scroll container resets to the top. Remember where the grid was scrolled when leaving it, and
+  // restore it when we come back — so the browser/mouse Back from the player lands on the same card
+  // instead of the top of the library. The scroller is App's <main> (the page's overflow-y-auto).
+  const scrollRef = useRef(0);
+  const scroller = () => document.querySelector("main");
+  useLayoutEffect(() => {
+    if (sub.view.kind === "grid" && scrollRef.current) {
+      const el = scroller();
+      if (el) el.scrollTop = scrollRef.current;
+    }
+  }, [sub.view.kind]);
+
   const browseQ = useInfiniteQuery({
     queryKey: ["plex-browse", library, dq, sort, show],
     enabled: !!library && sub.view.kind === "grid",
@@ -66,6 +79,7 @@ export default function PlexBrowse({ q, library, show, sort }: Props) {
   }, [hasNextPage, isFetchingNextPage, fetchNextPage]);
 
   function onCard(card: PlexCard) {
+    scrollRef.current = scroller()?.scrollTop ?? 0; // remember grid position for the trip back
     if (card.type === "show") sub.open({ kind: "show", id: card.id });
     else sub.open({ kind: "player", id: card.id });
   }