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/api/bindings.ts

@@ -2538,7 +2538,19 @@ export type User = { id: string; name: string; serverId: string; primaryImageTag
 /**
  * User-specific data for an item (playback state, favorites, etc.)
  */
-export type UserData = { playbackPositionTicks?: number | null; isPlayed?: boolean | null; isFavorite?: boolean | null; playCount?: number | null; lastPlayedDate?: string | null; playbackContextType?: string | null; playbackContextId?: string | null }
+export type UserData = { 
+/**
+ * Legacy Jellyfin resume position in ticks. Being replaced by
+ * `playback_position_ms`; dual-carried while the frontend migrates
+ * (docs/specs/frontend-domain-model.md). New code should read the ms field.
+ */
+playbackPositionTicks?: number | null; 
+/**
+ * Resume position in milliseconds — the neutral replacement for
+ * `playback_position_ticks`. Populated from ticks by the mapping; the
+ * frontend never divides ticks itself.
+ */
+playbackPositionMs?: number | null; isPlayed?: boolean | null; isFavorite?: boolean | null; playCount?: number | null; lastPlayedDate?: string | null; playbackContextType?: string | null; playbackContextId?: string | null }
 /**
  * User info returned to frontend
  */