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/utils/duration.ts

@@ -1,21 +1,21 @@
 /**
- * Duration formatting utilities
+ * Duration formatting utilities.
  *
- * Jellyfin uses "ticks" for duration where 10,000,000 ticks = 1 second
+ * Durations are milliseconds — the app's neutral time unit. The backend has
+ * already converted any provider unit (e.g. Jellyfin ticks) before it reaches
+ * the frontend, so no tick arithmetic lives here.
  */
 
 /**
- * Convert Jellyfin ticks to formatted duration string
- * @param ticks Duration in Jellyfin ticks (10M ticks = 1 second)
+ * Convert a millisecond duration to a formatted string.
+ * @param ms Duration in milliseconds
  * @param format Format type: "mm:ss" (default) or "hh:mm:ss"
- * @returns Formatted duration string or empty string if no ticks
+ * @returns Formatted duration string or empty string if no duration
  */
-export function formatDuration(ticks?: number | null, format: "mm:ss" | "hh:mm:ss" = "mm:ss"): string {
-  if (!ticks) return "";
+export function formatDuration(ms?: number | null, format: "mm:ss" | "hh:mm:ss" = "mm:ss"): string {
+  if (!ms) return "";
 
-  // Jellyfin uses 10,000,000 ticks per second
-  const TICKS_PER_SECOND = 10000000;
-  const totalSeconds = Math.floor(ticks / TICKS_PER_SECOND);
+  const totalSeconds = Math.floor(ms / 1000);
 
   if (format === "hh:mm:ss") {
     const hours = Math.floor(totalSeconds / 3600);