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/lib/format.ts

@@ -70,6 +70,15 @@ export function formatDuration(sec: number | null): string {
   return h > 0 ? `${h}:${pad(m)}:${pad(s)}` : `${m}:${pad(s)}`;
 }
 
+/** Compact human runtime ("1h 30m", "45m") from seconds — used for Plex movie/episode lengths.
+ * Empty string for a missing/zero duration (nothing to show). */
+export function formatRuntime(sec?: number | null): string {
+  if (!sec) return "";
+  const h = Math.floor(sec / 3600);
+  const m = Math.floor((sec % 3600) / 60);
+  return h ? `${h}h ${m}m` : `${m}m`;
+}
+
 // Human label for a canonical quota-action key (see backend app.quota.QuotaAction). Resolved
 // from i18n (quotaActions.<key>) so it's translated in all UI languages; falls back to the raw
 // key for any unmapped/legacy value.