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/PlexBrowse.tsx

@@ -26,6 +26,7 @@ import {
   type PlexSeasonDetail,
   type PlexUnifiedResult,
 } from "../lib/api";
+import { formatRuntime } from "../lib/format";
 import { useDebounced } from "../lib/useDebounced";
 import { useHistorySubview } from "../lib/history";
 import { DetailCustomizeMenu, Filterable, useArtBackdrop, useDetailPrefs } from "../lib/plexDetailUi";
@@ -67,13 +68,6 @@ type Props = {
 
 const PAGE = 40;
 
-function dur(n?: number | null): string {
-  if (!n) return "";
-  const h = Math.floor(n / 3600);
-  const m = Math.floor((n % 3600) / 60);
-  return h ? `${h}h ${m}m` : `${m}m`;
-}
-
 export default function PlexBrowse({
   q,
   onClearSearch,
@@ -483,7 +477,7 @@ function PlexPosterCard({
         style={{ background: "color-mix(in srgb, var(--card) 60%, transparent)" }}
       >
         <div className="text-sm font-medium line-clamp-2 leading-tight hover:text-accent">{card.title}</div>
-        <div className="text-xs text-muted">{[card.year, dur(card.duration_seconds)].filter(Boolean).join(" · ")}</div>
+        <div className="text-xs text-muted">{[card.year, formatRuntime(card.duration_seconds)].filter(Boolean).join(" · ")}</div>
       </div>
     </div>
   );
@@ -526,7 +520,8 @@ function PlexInfoView({
             onPlay={onPlay}
             onPlayItem={onPlayItem}
             onFilter={onFilter}
-            onStateChange={() => q.refetch()}
+            // No onStateChange: PlexInfo.setState already invalidates ["plex-item", id] (this very
+            // query), so q refetches automatically — a manual refetch here just double-fetched.
           />
         </Suspense>
       )}
@@ -845,7 +840,7 @@ function EpisodeCard({
             {!withShowTitle && <span className="text-muted mr-1">{ep.episode_number}.</span>}
             {ep.title}
           </div>
-          <div className="text-[11px] text-muted mt-0.5">{dur(ep.duration_seconds)}</div>
+          <div className="text-[11px] text-muted mt-0.5">{formatRuntime(ep.duration_seconds)}</div>
         </div>
         {onAdd && (
           <button