feat(emby): add Emby as a MediaBrowser backend alongside Jellyfin

Emby is Jellyfin's upstream ancestor and speaks a near-identical MediaBrowser
API, so the existing Jellyfin stack is parameterised by a `MediaBrowserDialect`
rather than forked. `JellyfinClient`, its auth service, endpoint discovery, LAN
discovery, and the add/edit connection screens all take the dialect and keep one
implementation; `MediaBackend.emby` and `ConnectionKind.emby` carry it through
the neutral models, the Drift `kind` discriminator, downloads, and caches.

Every divergence below was measured against a live Emby 4.9.5 server, not
inferred from documentation, and each is documented at its capability getter.
Jellyfin's request strings stay byte-identical so nothing about its behaviour
changes.

Routes and auth
- Emby only accepts the pre-10.9 user-scoped item routes (`/Users/{id}/Items/…`,
  `/Users/{id}/PlayedItems/…`, `/Users/{id}/FavoriteItems/…`); the unprefixed
  forms Jellyfin 10.11 added return 404.
- The API is also served under a legacy `/emby` prefix, and both dialects accept
  the token as `X-Emby-Token` or `api_key=`.
- Emby answers only its own LAN discovery datagram ("who is EmbyServer?") and
  ignores Jellyfin's; its default HTTPS port is 8920.
- No `/QuickConnect` route exists, so Quick Connect stays Jellyfin-only.

Row fields Emby withholds
- `ProductionYear`, `OfficialRating`, `PremiereDate` and `DateCreated` are absent
  from list rows unless named in `Fields`, which would otherwise strip the year
  and age-rating badge from every card in the app.
- `UserData.LastPlayedDate` never appears on a list row under `Fields=UserData`,
  `EnableUserData=true` or the user-scoped `Ids=` form — only on the single-item
  detail route, or when the Emby-specific `UserDataLastPlayedDate` token is
  requested. Without it every recency-ordered surface silently degrades to
  library-add time, and `JellyfinApiCache.applyWatchState` stamps
  `DateTime.now()` on watched rows, so an offline watch-state pull would rewrite
  the cached play time of everything it walked.

Continue Watching and Next Up
- Emby computes Next Up per series only: the library-wide `/Shows/NextUp` query
  returns nothing under every parameter combination tried. The shelf is
  therefore reconstructed from a played-episode recency scan plus one
  `/Shows/NextUp?SeriesId=` per distinct series, bounded by a shared wall clock
  that covers the scan as well — per-request timeouts cannot bound the pass
  because `MediaServerHttpClient` times the connect and receive phases
  independently. Rows are stamped with their series' newest play from the same
  response that ordered them, so no per-series enrichment request is needed.
- `/Shows/NextUp` ignores `NextUpDateCutoff`, and no server-side played-date
  filter exists to delegate to (`MinDatePlayed` and `MinDateLastPlayed` are
  ignored; `MinDateLastSaved`, `MinDateCreated` and `MinPremiereDate` filter
  unrelated dates), so the 365-day window is applied to the scanned dates.
- The resume route returns items with no saved position, including plain next
  episodes, so the Emby resume leg reads from `/Items?Filters=IsResumable`.
- Emby is ahead of Jellyfin in one place: `/Users/{id}/Items/{id}/HideFromResume`
  makes Continue Watching removal a real capability.

Everything else
- `/Sessions/Playing` and `/Sessions/Playing/Progress` reject a body with no
  `PlaySessionId` (HTTP 400), so playback reporting always sends one.
- Passing any `MediaTypes` value to the playlist query returns an empty list.
- There is no aggregate `/Items/Filters` route; the four filter facets are
  reassembled from `/Genres`, `/OfficialRatings`, `/Studios` and `/Tags`.
- Metadata writes take name-pair lists (`Genres: [{'Name': 'Action'}]`); the
  plain string array is accepted and then silently discarded.
- Custom artwork uploads must be base64 text, not raw bytes — which was broken
  for Jellyfin too and is fixed for both.
- Trickplay, media segments and lyrics 404 on Emby, so scrub previews are absent
  and intro/credit markers fall back to chapter names.

Verified against a local Emby 4.9.5 and a Jellyfin 10.11.11 control server:
onboarding, browse, detail, playable stream URLs serving real bytes, subtitle
sidecars, watch-state write and restore, hubs, cross-server aggregation and
search across both backends simultaneously.

lib/metadata_edit/jellyfin_metadata_edit_adapter.dart

@@ -13,7 +13,7 @@ class JellyfinMetadataEditAdapter extends MetadataEditAdapter {
   JellyfinMetadataEditAdapter(this.client);
 
   @override
-  MediaBackend get backend => MediaBackend.jellyfin;
+  MediaBackend get backend => client.backend;
 
   @override
   MediaServerClient get mediaClient => client;
@@ -26,7 +26,7 @@ class JellyfinMetadataEditAdapter extends MetadataEditAdapter {
   Future<MetadataEditDraft> load(MediaItem item) async {
     final raw = await client.fetchEditableMetadataItem(item.id);
     if (raw == null) {
-      throw StateError('Editable Jellyfin metadata item is unavailable');
+      throw StateError('Editable MediaBrowser metadata item is unavailable');
     }
     final values = <String, Object?>{};
     _writeCommonValues(values, raw, item);
@@ -56,8 +56,8 @@ class JellyfinMetadataEditAdapter extends MetadataEditAdapter {
     final dto = Map<String, dynamic>.from(raw);
 
     dto['ProviderIds'] = _stringMap(dto['ProviderIds']);
-    dto['Tags'] = metadataStringList(dto['Tags']);
-    dto['Genres'] = metadataStringList(dto['Genres']);
+    dto['Tags'] = _namedStringList(dto, 'Tags', 'TagItems');
+    dto['Genres'] = _namedStringList(dto, 'Genres', 'GenreItems');
     dto['People'] = _mapList(dto['People']);
     dto['Studios'] = _mapList(dto['Studios']);
     dto['LockedFields'] = metadataStringList(dto['LockedFields']);
@@ -102,6 +102,17 @@ class JellyfinMetadataEditAdapter extends MetadataEditAdapter {
     }
     if (peopleChanged) dto['People'] = people;
 
+    // Emby ignores the plain `Genres`/`Tags` string lists on `POST /Items/{id}`
+    // and reads the `GenreItems`/`TagItems` name-pair arrays instead. The
+    // fetched DTO carries the *old* pairs, so mirroring unconditionally (not
+    // only when the field changed) is what keeps a save from silently
+    // reinstating the previous genres. See
+    // [MediaBrowserDialect.metadataWritesUseNamePairLists].
+    if (client.dialect.metadataWritesUseNamePairLists) {
+      dto['GenreItems'] = _toNamePairs(dto['Genres']);
+      dto['TagItems'] = _toNamePairs(dto['Tags']);
+    }
+
     final success = await client.updateMetadataItem(draft.sourceItem.id, dto);
     if (success) {
       draft.extras['raw'] = dto;
@@ -178,12 +189,12 @@ class JellyfinMetadataEditAdapter extends MetadataEditAdapter {
         ? metadataFirstString(raw['Taglines'])
         : item.tagline ?? '';
     values['summary'] = raw['Overview'] as String? ?? item.summary ?? '';
-    values['genre'] = metadataStringList(raw['Genres']);
+    values['genre'] = _namedStringList(raw, 'Genres', 'GenreItems');
     values['director'] = _peopleByType(raw['People'], 'Director');
     values['writer'] = _peopleByType(raw['People'], 'Writer');
     values['producer'] = _peopleByType(raw['People'], 'Producer');
     values['country'] = metadataStringList(raw['ProductionLocations']);
-    values['label'] = metadataStringList(raw['Tags']);
+    values['label'] = _namedStringList(raw, 'Tags', 'TagItems');
   }
 
   void _writeArtworkValues(Map<String, Object?> values, MediaItem item) {
@@ -242,6 +253,18 @@ List<String> _nameList(Object? value) {
       .toList();
 }
 
+/// Effective value of a `Genres`/`Tags` style field, preferring the plain string
+/// array and falling back to its `…Items` name-pair sibling.
+///
+/// Emby never returns the plain `Tags` array at all — only `TagItems`, whatever
+/// `Fields` the request asks for (measured on Emby 4.9.5). Reading the plain key
+/// alone would show an empty tag editor for an item that has tags and, worse,
+/// write that emptiness back on the next save.
+List<String> _namedStringList(Map<String, dynamic> dto, String key, String pairKey) {
+  final plain = metadataStringList(dto[key]);
+  return plain.isNotEmpty ? plain : _nameList(dto[pairKey]);
+}
+
 List<String> _peopleByType(Object? value, String type) {
   return _mapList(value)
       .where((person) => (person['Type'] as String?)?.toLowerCase() == type.toLowerCase())
@@ -267,6 +290,13 @@ List<Map<String, dynamic>> _replaceNamePairs(List<Map<String, dynamic>> existing
   return names.map((name) => _preserveNamedMap(existing, used, name)).toList();
 }
 
+/// Project a `Genres`/`Tags` string list into the `[{'Name': …}]` shape Emby
+/// requires on write. The server assigns the `Id` for a new entry, so omitting
+/// it is correct — it resolves an existing tag by name and creates one when
+/// there is no match.
+List<Map<String, dynamic>> _toNamePairs(Object? names) =>
+    metadataStringList(names).map((name) => <String, dynamic>{'Name': name}).toList();
+
 Map<String, dynamic> _preserveNamedMap(
   List<Map<String, dynamic>> existing,
   Set<int> used,