perf(detail): paint a show before its on-deck episode is looked up

Jellyfin has no equivalent of Plex's bundled `?includeOnDeck=1`, so a show
detail open chained `/Shows/NextUp` behind the item fetch and the screen sat
on a spinner for both round trips. The second one is not needed to paint:
everything except the play button's episode label comes from the item.

`fetchItemWithOnDeck` now takes an `onItemReady` callback and invokes it as
soon as the item is known, when that is strictly before on-deck settles.
Plex returns both together and never invokes it.

Phone and desktop only. TV keeps its own reveal gate — `_isTvDetailReadyToReveal`
holds the foreground at opacity 0 until extras, related hubs, seasons and the
first episode page have all loaded, and those still run after the on-deck
lookup settles, so TV sees no change. Both halves are pinned by tests.

Measured on a remote Jellyfin server, 15 interleaved show-detail opens per
version: time to content 1264ms -> 1042ms (-18%), with the rest of the load
unchanged.

Seasons and extras deliberately still start after the whole lookup settles.
Starting them at the early paint measured worse (time to settled +21%)
because they contend with the on-deck request instead of overlapping it — the
same reason `/Shows/NextUp` is not fired in parallel with the item fetch.
That trade-off is also why TV was left alone rather than being unblocked by
moving those loads earlier.

Two ordering hazards the early paint introduces, both covered by
`media_detail_screen_test.dart`:

- The early call must not write on-deck. `_loadFullMetadata` runs again after
  playback, and clearing there would blank the play button for the length of
  the round trip. `onDeckSettled` marks the authoritative write, so a reload
  that finds the series finished still clears it.
- A settled empty on-deck must not drop the episode-derived fallback that
  `_ensureFallbackOnDeckEpisode` supplies.

close #1784

lib/services/jellyfin_client/parts/browse.dart

@@ -613,17 +613,22 @@ mixin _JellyfinBrowseMethods on _JellyfinClientInternals {
   /// OnDeck semantics: returns the resume episode when one exists, or S1E1
   /// when the user hasn't started. Movies and other kinds short-circuit.
   ///
-  /// Deliberately still chained rather than fired in parallel off a caller
-  /// kind hint: measured against a remote server that saved nothing, because
-  /// the two requests contend rather than overlap (`/Shows/NextUp` went from
-  /// 380ms alone to 1395ms beside the detail fetch), and it would cost a
-  /// wasted request on every movie whose hint was absent or wrong.
+  /// The chain stays sequential — firing both together measured no better,
+  /// because the requests contend rather than overlap (`/Shows/NextUp` went
+  /// from 380ms alone to 1395ms beside the detail fetch) and a movie would pay
+  /// for a request it can never use. Instead the item is handed to
+  /// [onItemReady] the moment it lands, so the caller can paint without
+  /// waiting for the on-deck round trip (#1784).
   @override
-  Future<({MediaItem? item, MediaItem? onDeckEpisode})> fetchItemWithOnDeck(String id) async {
+  Future<({MediaItem? item, MediaItem? onDeckEpisode})> fetchItemWithOnDeck(
+    String id, {
+    void Function(MediaItem item)? onItemReady,
+  }) async {
     final item = await fetchItem(id);
     if (item == null || item.kind != MediaKind.show) {
       return (item: item, onDeckEpisode: null);
     }
+    onItemReady?.call(item);
     final nextUp = await _safeFetchItemsArray('/Shows/NextUp', {
       'seriesId': id,
       'userId': connection.userId,

lib/services/plex_client.dart

@@ -3769,10 +3769,14 @@ class PlexClient
   }
 
   /// Full item with on-deck episode from a single `/library/metadata/{id}`
-  /// round-trip. Implements [MediaServerClient.fetchItemWithOnDeck];
-  /// Jellyfin has no analogous endpoint and returns onDeck=null there.
+  /// round-trip. Implements [MediaServerClient.fetchItemWithOnDeck]. Both
+  /// halves arrive together, so there is no window in which the item is known
+  /// and on-deck is not — `onItemReady` is intentionally never invoked.
   @override
-  Future<({MediaItem? item, MediaItem? onDeckEpisode})> fetchItemWithOnDeck(String id) async {
+  Future<({MediaItem? item, MediaItem? onDeckEpisode})> fetchItemWithOnDeck(
+    String id, {
+    void Function(MediaItem item)? onItemReady,
+  }) async {
     try {
       final result = await getMetadataWithImagesAndOnDeck(id, shouldFallback: _shouldFallbackPlexItemLookup);
       final itemDto = result['metadata'] as PlexMetadataDto?;