chore(plex): dedup frontend formatters + LS key, drop dead wasPlaying pref

- format.ts: new formatRuntime() ("1h 30m"); PlexBrowse dur() + PlexInfo
  fmtDur() were byte-identical copies, now both call it.
- PlexPlayer fmt() delegates to formatDuration (keeps the NaN/negative clamp
  the live media clock needs); local reimplementation gone.
- Register LS.plexPlayerPrefs; PlexPlayer uses it instead of a bare string.
- Remove the dead PlexPlayerPrefs.wasPlaying field (written on play/pause, never
  read) + its two patchPrefs writes.
- PlexBrowse item page: drop the redundant onStateChange={() => q.refetch()} —
  PlexInfo.setState already invalidates ["plex-item", id], so q refetches itself.

frontend/src/components/PlexInfo.tsx

@@ -3,6 +3,7 @@ import { useTranslation } from "react-i18next";
 import { useQueryClient } from "@tanstack/react-query";
 import { Check, ExternalLink, FolderPlus, ListPlus, Play, RotateCcw, Star, X } from "lucide-react";
 import { api, type PlexFilters, type PlexItemDetail } from "../lib/api";
+import { formatRuntime } from "../lib/format";
 import { DetailCustomizeMenu, Filterable, PrefToggle, useArtBackdrop, useDetailPrefs } from "../lib/plexDetailUi";
 import PlexCollectionEditor from "./PlexCollectionEditor";
 import PlexPlaylistAdd from "./PlexPlaylistAdd";
@@ -28,13 +29,6 @@ type Props = {
   library?: string;
 };
 
-function fmtDur(s?: number | null): string {
-  if (!s) return "";
-  const h = Math.floor(s / 3600);
-  const m = Math.floor((s % 3600) / 60);
-  return h ? `${h}h ${m}m` : `${m}m`;
-}
-
 export default function PlexInfo({
   detail,
   variant,
@@ -170,7 +164,7 @@ export default function PlexInfo({
                   {detail.content_rating}
                 </Filterable>
               )}
-              {detail.duration_seconds ? <span className={mutedCls}>{fmtDur(detail.duration_seconds)}</span> : null}
+              {detail.duration_seconds ? <span className={mutedCls}>{formatRuntime(detail.duration_seconds)}</span> : null}
               {detail.studio && (
                 <Filterable
                   className={mutedCls}