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/TrackList.svelte

@@ -283,7 +283,7 @@
 
           <!-- Duration -->
           <div class="text-gray-400 text-right">
-            {formatDuration(track.runTimeTicks)}
+            {formatDuration(track.durationMs)}
           </div>
 
           <!-- Download Button Placeholder -->
@@ -421,7 +421,7 @@
             </p>
           </div>
           <div class="text-gray-400 text-sm {showDownload ? 'mr-20' : 'mr-12'}">
-            {formatDuration(track.runTimeTicks)}
+            {formatDuration(track.durationMs)}
           </div>
         </button>