feat(players): keep a video playing in the background on Brave iOS; repeat on the watch page

On iPad Brave, a shared video stopped the moment the browser left the
foreground, even with Brave's "Enable Background Audio" on, while an mp3
kept playing. Brave iOS keeps a web <video> going by re-playing one the OS
paused (brave-core ios/browser/web/media/resources/media_backgrounding.ts),
but it only hooks a <video> present at document start or added as a
MutationObserver addedNode that IS a video. React inserts a player's
<video> inside its freshly mounted wrapper in one go, so Brave never saw it.

lib/browserBackgroundPlay re-inserts a mounted <video> at its own position,
once per element: it becomes an added node, the DOM React manages does not
change, and playback is not interrupted. The watch page, the downloads
library player and the Plex player hand their <video> to it. Measured on the
owner's iPad (Brave) with a prototype of the same idea: the video kept
playing in another app and with the screen locked. The YouTube modal is not
ours to change: its <video> lives in YouTube's cross-origin iframe, and
nothing of ours pauses it on backgrounding.

The prototype's two audio-element variants (a muted video following an
<audio>, and an <audio> taking over on hide) went silent in the background on
the iPad and are removed.

The watch page gets a Repeat toggle, on by default and remembered per
browser (LS.watchRepeat; the page has no account): it sets `loop` on the
<video> or the audio-only <audio>.

frontend/src/components/LibraryPlayer.tsx

@@ -3,6 +3,7 @@ import { useTranslation } from "react-i18next";
 import Modal from "./ui/Modal";
 import { api, type DownloadJob } from "../lib/api";
 import { jobTitle } from "./downloadRow";
+import { exposeVideoToBackgroundPlay } from "../lib/browserBackgroundPlay";
 
 // E7 S4b — 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
@@ -47,6 +48,7 @@ export default function LibraryPlayer({ job, onClose }: { job: DownloadJob; onCl
         </div>
       ) : (
         <video
+          ref={exposeVideoToBackgroundPlay}
           src={fileUrl}
           poster={poster}
           controls