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/storage.ts

@@ -32,6 +32,7 @@ export const LS = {
   plexFilters: "siftlode.plexFilters",
   plexQ: "siftlode.plexQ",
   plexPlaylistLayout: "siftlode.plexPlaylistLayout",
+  plexPlayerPrefs: "siftlode.plexPlayerPrefs",
   notifHistory: "siftlode.notifications",
   notifSettings: "siftlode.notifSettings",
   onboardingDismissed: "siftlode.onboarding.dismissed",