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/providers/multi_server_provider.dart

@@ -222,7 +222,7 @@ class MultiServerProvider extends ChangeNotifier with DisposableChangeNotifierMi
 
   /// Whether at least one online server is a Plex server. Used to gate
   /// Plex-only chrome (server-activities popover, conflict-resolution
-  /// helpers) so they don't render against a Jellyfin-only profile.
+  /// helpers) so it doesn't render against a MediaBrowser-only profile.
   bool get hasOnlinePlexServers => onlineServerIds.any((id) => _serverManager.getPlexClient(ServerId(id)) != null);
 
   /// Visibility-filtered server ids whose latest health probe was rejected
@@ -263,10 +263,9 @@ class MultiServerProvider extends ChangeNotifier with DisposableChangeNotifierMi
 
   /// Check all online servers for DVR/Live TV availability. Plex servers
   /// expose `/livetv/dvrs` (one entry per configured DVR with its own
-  /// lineup); Jellyfin servers expose `/LiveTv/Channels` with a single
+  /// lineup); MediaBrowser servers expose `/LiveTv/Channels` with a single
   /// flat channel list per server (synthesized into one [LiveTvServerInfo]
-  /// with `dvrKey: 'jellyfin'` so the rest of the UI's per-DVR loop works
-  /// uniformly).
+  /// whose backend-derived `dvrKey` keeps the UI's per-DVR identity stable).
   Future<void> checkLiveTvAvailability() async {
     if (isDisposed) return;
     final generation = ++_liveTvCheckGeneration;
@@ -285,9 +284,11 @@ class MultiServerProvider extends ChangeNotifier with DisposableChangeNotifierMi
             newLiveTvServers.add(LiveTvServerInfo(serverId: serverId, dvrKey: dvr.key, lineup: dvr.lineup, dvrs: dvrs));
           }
         } else if (await liveTv.isAvailable()) {
-          // Jellyfin: no per-DVR partitioning; synthesize a single entry so
-          // the rest of the UI's per-DVR loop works uniformly.
-          newLiveTvServers.add(LiveTvServerInfo(serverId: serverId, dvrKey: 'jellyfin', lineup: null, dvrs: const []));
+          // MediaBrowser: no per-DVR partitioning; synthesize a single entry
+          // so the rest of the UI's per-DVR loop works uniformly.
+          newLiveTvServers.add(
+            LiveTvServerInfo(serverId: serverId, dvrKey: genericClient.backend.id, lineup: null, dvrs: const []),
+          );
         }
       } catch (e) {
         appLogger.d('LiveTV check failed for server $serverId', error: e);