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.test.ts

@@ -72,7 +72,7 @@ describe("TrackList", () => {
       artists: ["Artist 1"],
       albumName: "Album 1",
       albumId: "album-1",
-      runTimeTicks: 1800000000, // 3 minutes
+      durationMs: 180000, // 3 minutes
       primaryImageTag: "tag1",
       indexNumber: 1,
     },
@@ -84,7 +84,7 @@ describe("TrackList", () => {
       artists: ["Artist 2"],
       albumName: "Album 2",
       albumId: "album-2",
-      runTimeTicks: 2400000000, // 4 minutes
+      durationMs: 240000, // 4 minutes
       primaryImageTag: "tag2",
       indexNumber: 2,
     },
@@ -96,7 +96,7 @@ describe("TrackList", () => {
       artists: ["Artist 3", "Artist 4"],
       albumName: "Album 3",
       albumId: "album-3",
-      runTimeTicks: 3000000000, // 5 minutes
+      durationMs: 300000, // 5 minutes
       indexNumber: 3,
     },
   ];
@@ -187,7 +187,7 @@ describe("TrackList", () => {
       const tracksWithoutDuration: MediaItem[] = [
         {
           ...mockTracks[0],
-          runTimeTicks: undefined,
+          durationMs: undefined,
         },
       ];