Restructure step 4: local library and media identity

- L1 (core, offline): release names with kinds (episode, movie, extra:
  OP/ED/Opening/Ending/NC/PV/OVA…), folder hints (season and name from
  parentheticals, ranges), one show across folders, gaps and next with
  skipped numbers, junk ignored, stable keys.
- L2 (app, device-owned cache): AniList match with synonyms and
  sequel walking, episode mapping across cours (AnimeFlow runs when
  signed in), art and episode titles, "Wrong show?" pins per show or
  folder, episode offsets; lazy, spaced requests, works offline.
- Files tab as a library: poster cards, show pages with banner, gap
  prompt, extras, movies, Wrong show? and Episode numbers.
- media.ref produced for local files, Jellyfin (provider ids) and links,
  carried in swaps and Up next; receivers name media from it.
- Hosting relays each Jellyfin item through its own server's login.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

apps/desktop/lib/src/library/media_refs.dart

@@ -0,0 +1,80 @@
+import 'package:sync_relay_core/core.dart';
+
+// `media.ref` (PROTOCOL.md "Media identity") for each source: what a media
+// is, independent of where its bytes come from. Local files get theirs
+// from the library (`ShowMatcher.refFor`, L1 + L2); these are the other
+// two: Jellyfin items by their provider ids, links best effort from their
+// name.
+
+/// A Jellyfin episode, movie or video: kind, series (or movie) title,
+/// season and episode as Jellyfin numbers them, and the AniList / TVDB ids
+/// its metadata plugins set ([series]' when given: an episode's own ids
+/// are the episode's, not the show's). Null for anything that is not
+/// played (series, seasons, folders).
+MediaRef? jellyfinMediaRef(JellyfinItem item, {JellyfinItem? series}) {
+  final kind = switch (item.kind) {
+    JellyfinItemKind.episode => 'episode',
+    JellyfinItemKind.movie || JellyfinItemKind.video => 'movie',
+    _ => null,
+  };
+  if (kind == null) return null;
+  final episode = kind == 'episode';
+  // An episode's Tvdb id names the episode; the show's comes from the
+  // series item. AniList ids are only ever set on shows and movies.
+  final owner = episode ? series : item;
+  final special = episode && item.parentIndexNumber == 0;
+  return MediaRef(
+    kind: special ? 'extra' : kind,
+    title: episode ? (item.seriesName ?? series?.name ?? item.name) : item.name,
+    anilistId: owner?.providerId('AniList'),
+    tvdbId: owner?.providerId('Tvdb'),
+    season: episode && !special ? item.parentIndexNumber : null,
+    ep: episode && !special ? item.indexNumber : null,
+  );
+}
+
+final _sxe = RegExp(r'S(\d{1,3})\s?E(\d{1,4})$', caseSensitive: false);
+
+/// A picked Jellyfin item from its card's lines alone ([title], and
+/// [subtitle] "Series · S1E6" for episodes, the year for movies): kind,
+/// title and numbers, no ids.
+MediaRef jellyfinPickRef(String title, String subtitle) {
+  final sub = subtitle.trim();
+  final cut = sub.lastIndexOf(' · ');
+  final label = cut < 0 ? sub : sub.substring(cut + 3).trim();
+  final m = _sxe.firstMatch(label);
+  if (m == null) return MediaRef(kind: 'movie', title: title);
+  final season = int.parse(m.group(1)!);
+  return MediaRef(
+    kind: season == 0 ? 'extra' : 'episode',
+    title: cut < 0 ? title : sub.substring(0, cut).trim(),
+    season: season == 0 ? null : season,
+    ep: season == 0 ? null : int.parse(m.group(2)!),
+  );
+}
+
+/// A link, best effort: its name read like a release name (`Show - 05`
+/// is an episode, `OP1` an extra); anything else a movie of that title.
+MediaRef linkMediaRef(String name) {
+  final p = parseReleaseName(name);
+  final title = p.series.trim().isEmpty ? name.trim() : p.series.trim();
+  return switch (p.kind) {
+    ReleaseKind.episode => MediaRef(
+        kind: 'episode',
+        title: title,
+        season: p.season,
+        ep: p.season == null ? null : p.episode,
+        absolute: p.season == null ? p.episode : null),
+    ReleaseKind.extra => MediaRef(kind: 'extra', title: title, season: p.season),
+    ReleaseKind.movie || ReleaseKind.other => MediaRef(kind: 'movie', title: title),
+  };
+}
+
+/// The episode line a ref gives (`S3 E13`, `E108`), or null (movies;
+/// extras keep the name's own marker).
+String? refEpisodeLine(MediaRef ref) {
+  if (ref.kind != 'episode') return null;
+  if (ref.season != null && ref.ep != null) return 'S${ref.season} E${ref.ep}';
+  final n = ref.absolute ?? ref.ep ?? ref.anilistEp;
+  return n == null ? null : 'E$n';
+}