feat(player): the OS media controls, the basic keys and resume for the library and shared-link players

Plex, library and /watch tell the operating system what plays (title, line, picture) and take its
play / pause / skip / seek through one hook (lib/mediaSession); the Plex player drives it with its own
seek and clock, so the lock screen shows the film's position, not the stream's. The YouTube player is
left alone: its iframe already publishes all of it, and the OS routes to the frame that plays.

The library and /watch players get the basic keys (lib/mediaKeys, the keymap's mediaPlayer rows) and
open where the viewer stopped (lib/resume): the library keeps the point on the server, per user and
library item (library_states, POST /api/downloads/{id}/progress, listed back on both listings), the
shared link in this browser. A started library row continues, its menu can start it over, and its
picture carries the cards' progress bar. The resume rule (too early / near the end = from the start)
is one function shared with the YouTube route.

The library dialog opens focused on its panel (Modal initialFocus "player"): on the ✕, Space closed
the film. While the media element itself has the focus, Space and the arrows stay its own controls'
— the browser acts on them whatever a listener does, and one Space played and paused.

frontend/src/components/LibraryPlayer.tsx

@@ -1,10 +1,15 @@
-import { useCallback, useMemo } from "react";
+import { useCallback, useMemo, useState } from "react";
 import { useTranslation } from "react-i18next";
+import { useQueryClient } from "@tanstack/react-query";
 import Modal from "./ui/Modal";
 import { api, type DownloadJob } from "../lib/api";
+import { qk } from "../lib/queryKeys";
 import { jobTitle } from "./downloadTitle";
 import { exposeVideoToBackgroundPlay } from "../lib/browserBackgroundPlay";
 import { useMediaLogRef } from "../lib/mediaLog";
+import { useMediaSession } from "../lib/mediaSession";
+import { useMediaKeys } from "../lib/mediaKeys";
+import { keptPosition, useResume } from "../lib/resume";
 
 // In-app player for a completed library item. We do NOT reuse PlayerModal (a cross-origin
 // YouTube IFrame shell) or PlexPlayer (a Plex HLS/session shell): both are bound to their own domain
@@ -14,8 +19,21 @@ import { useMediaLogRef } from "../lib/mediaLog";
 // browser-native container (mp4/H.264/AAC + m4a/mp3), so native playback (seek via the range-aware
 // /file endpoint, volume, fullscreen, speed) works with zero transcode; an uploaded media item may be
 // any codec (best-effort playback).
-export default function LibraryPlayer({ job, onClose }: { job: DownloadJob; onClose: () => void }) {
+//
+// It opens where the viewer stopped (`job.position_seconds`, from the library listing) unless
+// `fromStart`, and keeps the point on the server as it plays (`lib/resume.ts`); the OS's media
+// controls and the basic keys drive it like the other players.
+export default function LibraryPlayer({
+  job,
+  fromStart = false,
+  onClose,
+}: {
+  job: DownloadJob;
+  fromStart?: boolean;
+  onClose: () => void;
+}) {
   const { t } = useTranslation();
+  const qc = useQueryClient();
   const fileUrl = useMemo(() => api.downloadFileUrl(job.id), [job.id]);
   // Video vs audio-only comes from the stored media_kind (the single source every sibling reads —
   // WatchPage/downloadRow/DownloadCenter) — NOT a re-derived container list, which would misclassify
@@ -32,16 +50,40 @@ export default function LibraryPlayer({ job, onClose }: { job: DownloadJob; onCl
   // the fallback. Same precedence as the library-row thumbnail.
   const poster = job.poster_url ? api.posterImageUrl(job.id) : (asset?.thumbnail_url ?? undefined);
   const logMedia = useMediaLogRef("library");
-  const videoRef = useCallback(
-    (el: HTMLVideoElement | null) => {
-      exposeVideoToBackgroundPlay(el);
+  // The element as STATE, so the session, the keys and the resume bind to it once it exists.
+  const [media, setMedia] = useState<HTMLMediaElement | null>(null);
+  const mediaRef = useCallback(
+    (el: HTMLMediaElement | null) => {
+      if (el instanceof HTMLVideoElement) exposeVideoToBackgroundPlay(el);
       logMedia(el);
+      setMedia(el);
     },
     [logMedia],
   );
 
+  const title = jobTitle(job);
+  useMediaSession(media, {
+    title,
+    artist: job.display_uploader ?? asset?.uploader ?? null,
+    artwork: poster ?? null,
+  });
+  useMediaKeys(media);
+  // The library row's bar follows the point at once — both listings this job may sit in — rather
+  // than after the next refetch, which may land before the closing beacon does.
+  const keepPoint = (position: number, duration: number, leaving: boolean) => {
+    const at = keptPosition(position, duration);
+    for (const key of [qk.downloads(), qk.downloadsShared()]) {
+      qc.setQueryData<DownloadJob[]>(key, (jobs) =>
+        jobs?.map((j) => (j.id === job.id ? { ...j, position_seconds: at } : j)),
+      );
+    }
+    if (leaving) api.saveDownloadProgressBeacon(job.id, position, duration);
+    else void api.saveDownloadProgress(job.id, position, duration).catch(() => {});
+  };
+  useResume(media, String(job.id), fromStart ? 0 : (job.position_seconds ?? 0), keepPoint);
+
   return (
-    <Modal initialFocus="close" title={jobTitle(job)} onClose={onClose} maxWidth="max-w-4xl">
+    <Modal initialFocus="player" title={title} onClose={onClose} maxWidth="max-w-4xl">
       {isAudio ? (
         <div className="flex flex-col items-center gap-4 py-2">
           {poster ? (
@@ -51,13 +93,13 @@ export default function LibraryPlayer({ job, onClose }: { job: DownloadJob; onCl
               className="w-40 h-40 rounded-xl object-cover border border-border"
             />
           ) : null}
-          <audio ref={logMedia} src={fileUrl} controls autoPlay className="w-full">
+          <audio ref={mediaRef} src={fileUrl} controls autoPlay className="w-full">
             {t("downloads.player.unsupported")}
           </audio>
         </div>
       ) : (
         <video
-          ref={videoRef}
+          ref={mediaRef}
           src={fileUrl}
           poster={poster}
           controls