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/EpisodeFocusView.svelte

@@ -86,9 +86,9 @@
     return null;
   });
 
-  function formatDuration(ticks?: number | null): string {
-    if (!ticks) return "";
-    const seconds = Math.floor(ticks / 10000000);
+  function formatDuration(ms?: number | null): string {
+    if (!ms) return "";
+    const seconds = Math.floor(ms / 1000);
     const hours = Math.floor(seconds / 3600);
     const minutes = Math.floor((seconds % 3600) / 60);
 
@@ -99,10 +99,10 @@
   }
 
   function getProgress(ep: MediaItem): number {
-    if (!ep.userData || !ep.runTimeTicks) {
+    if (!ep.userData || !ep.durationMs) {
       return 0;
     }
-    return ((ep.userData.playbackPositionTicks ?? 0) / ep.runTimeTicks) * 100;
+    return ((ep.userData.playbackPositionMs ?? 0) / ep.durationMs) * 100;
   }
 
   function handlePlay() {
@@ -116,7 +116,7 @@
   const episodeLabel = $derived(
     `S${episode.parentIndexNumber || 1}E${episode.indexNumber || 1}`
   );
-  const duration = $derived(formatDuration(episode.runTimeTicks));
+  const duration = $derived(formatDuration(episode.durationMs));
   const progress = $derived(getProgress(episode));
 </script>