domain: catalog frontend off Jellyfin ticks -> milliseconds (phase 3a)

The catalog surface now speaks milliseconds, the app's neutral time unit.
Ticks no longer reach library/home components.

Rust:
- UserData gains playback_position_ms (dual-carry), populated from ticks
  at the offline mapping seam via domain::ticks_to_ms.

Frontend:
- formatDuration(duration.ts) and the two local copies now take ms, not
  ticks; all callers pass item.durationMs.
- Progress bars (EpisodeRow, EpisodeFocusView, MediaCard, LibraryListView)
  compute playbackPositionMs / durationMs — unit-consistent, no tick math.
- PlaylistDetailView totalDuration sums durationMs.
- duration.test.ts + TrackList.test.ts fixtures updated to ms.

Deferred: player/session/reporting tick math (Queue, SessionCard,
RemoteControls, playbackReporting, playerEvents) — those cross the
storage/Jellyfin command boundary in ticks and need command-signature
changes (phase 3b). Display {item.type} badge -> kind label (phase 4).

Rust 456, frontend 644, check + check:boundary clean.

src/lib/components/library/EpisodeRow.svelte

@@ -38,13 +38,13 @@
   const downloadProgress = $derived(downloadInfo?.progress || 0);
 
   const progress = $derived(() => {
-    if (!episode.userData || !episode.runTimeTicks) {
+    if (!episode.userData || !episode.durationMs) {
       return 0;
     }
-    return ((episode.userData.playbackPositionTicks ?? 0) / episode.runTimeTicks) * 100;
+    return ((episode.userData.playbackPositionMs ?? 0) / episode.durationMs) * 100;
   });
 
-  const duration = $derived(formatDuration(episode.runTimeTicks));
+  const duration = $derived(formatDuration(episode.durationMs));
   const episodeNumber = $derived(episode.indexNumber || 0);
 </script>