domain: delete dead jellyfinFieldMapping; scope playbackUnits to session boundary (phase 4e)

jellyfinFieldMapping.ts (SORT_FIELD_MAP friendly->Jellyfin sort names) had
zero consumers — sort code passes raw Jellyfin field names directly — so it
and its test are deleted.

playbackUnits.ts can't be removed: its tick<->seconds helpers are still the
correct converters for the remote Jellyfin *session* boundary
(SessionInfo.playState.positionTicks, NowPlayingItem.runTimeTicks), which
legitimately arrives in ticks. Documented that narrowed role; formatTime/
calculateProgress remain neutral seconds-based presentation helpers.

Note (out of scope): sortBy still passes raw Jellyfin field names
("SortName", "CommunityRating") — a separate sort-taxonomy leak that would
need its own Rust SortKey, like the search-scope work.

Frontend 626 tests (jellyfinFieldMapping's 18 removed with it), check clean.

src/lib/utils/playbackUnits.ts

@@ -1,29 +1,31 @@
 /**
- * Playback unit conversion utilities
+ * Playback unit utilities.
  *
- * Jellyfin uses "ticks" for time values where 10 million ticks = 1 second.
- * This module provides type-safe conversion functions to eliminate magic numbers
- * and prevent conversion bugs across the codebase.
+ * The app's own media model speaks milliseconds (see the domain migration in
+ * docs/specs/frontend-domain-model.md), so catalog code does NOT use the tick
+ * helpers here. Ticks survive only at the **remote Jellyfin session boundary** —
+ * `SessionInfo.playState.positionTicks` and `NowPlayingItem.runTimeTicks` arrive
+ * straight from a live Jellyfin session and are converted here for display.
+ * `formatTime`/`calculateProgress` are plain seconds-based presentation helpers.
  */
 
 /**
- * Number of Jellyfin ticks per second (10 million)
+ * Number of Jellyfin ticks per second (10 million).
+ *
+ * Only for the remote-session boundary (see module doc); catalog durations are
+ * milliseconds and never touch this.
  */
 export const TICKS_PER_SECOND = 10_000_000;
 
 /**
- * Convert seconds to Jellyfin ticks
- * @param seconds - Time in seconds (e.g., 90.5 for 1 minute 30.5 seconds)
- * @returns Time in Jellyfin ticks
+ * Convert seconds to Jellyfin ticks. Remote-session boundary only.
  */
 export function secondsToTicks(seconds: number): number {
   return Math.floor(seconds * TICKS_PER_SECOND);
 }
 
 /**
- * Convert Jellyfin ticks to seconds
- * @param ticks - Time in Jellyfin ticks
- * @returns Time in seconds
+ * Convert Jellyfin session ticks to seconds. Remote-session boundary only.
  */
 export function ticksToSeconds(ticks: number | null | undefined): number {
   return (ticks ?? 0) / TICKS_PER_SECOND;