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.

test/test_helpers/backend_client_fixtures.dart

@@ -2,11 +2,15 @@ import 'package:http/http.dart' as http;
 import 'package:http/testing.dart';
 import 'package:plezy/connection/connection.dart';
 import 'package:plezy/media/ids.dart';
+import 'package:plezy/media/media_browser_dialect.dart';
 import 'package:plezy/models/plex/plex_config.dart';
 import 'package:plezy/services/jellyfin_client.dart';
 import 'package:plezy/services/plex_client.dart';
 import 'package:plezy/utils/active_client_scope.dart';
 
+/// A MediaBrowser-family connection fixture. Defaults to the Jellyfin dialect;
+/// pass `dialect: MediaBrowserDialect.emby` (or use [testEmbyConnection]) to
+/// exercise the Emby routes.
 JellyfinConnection testJellyfinConnection({
   String machineId = 'srv-1',
   String userId = 'user-1',
@@ -21,6 +25,7 @@ JellyfinConnection testJellyfinConnection({
   ConnectionStatus status = ConnectionStatus.unknown,
   DateTime? createdAt,
   DateTime? lastAuthenticatedAt,
+  MediaBrowserDialect dialect = MediaBrowserDialect.jellyfin,
 }) {
   return JellyfinConnection(
     id: id ?? '$machineId/$userId',
@@ -32,6 +37,7 @@ JellyfinConnection testJellyfinConnection({
     userName: userName,
     accessToken: accessToken,
     deviceId: deviceId,
+    dialect: dialect,
     isAdministrator: isAdministrator,
     status: status,
     createdAt: createdAt ?? DateTime.utc(2024),
@@ -39,6 +45,42 @@ JellyfinConnection testJellyfinConnection({
   );
 }
 
+/// Emby-dialect twin of [testJellyfinConnection]. Same field defaults so a
+/// suite can be parameterized over both dialects and assert only the route
+/// differences.
+JellyfinConnection testEmbyConnection({
+  String machineId = 'srv-1',
+  String userId = 'user-1',
+  String? id,
+  String baseUrl = 'https://emby.example.com',
+  List<String>? baseUrls,
+  String serverName = 'Home',
+  String userName = 'User',
+  String accessToken = 'token',
+  String deviceId = 'device-1',
+  bool isAdministrator = false,
+  ConnectionStatus status = ConnectionStatus.unknown,
+  DateTime? createdAt,
+  DateTime? lastAuthenticatedAt,
+}) {
+  return testJellyfinConnection(
+    machineId: machineId,
+    userId: userId,
+    id: id,
+    baseUrl: baseUrl,
+    baseUrls: baseUrls,
+    serverName: serverName,
+    userName: userName,
+    accessToken: accessToken,
+    deviceId: deviceId,
+    isAdministrator: isAdministrator,
+    status: status,
+    createdAt: createdAt,
+    lastAuthenticatedAt: lastAuthenticatedAt,
+    dialect: MediaBrowserDialect.emby,
+  );
+}
+
 PlexConfig testPlexConfig({
   String baseUrl = 'https://plex.example.com',
   String? token = 'token',
@@ -81,6 +123,22 @@ JellyfinClient testJellyfinClient({
   );
 }
 
+/// Emby-dialect twin of [testJellyfinClient] — same `JellyfinClient` class, an
+/// Emby connection underneath.
+JellyfinClient testEmbyClient({
+  JellyfinConnection? connection,
+  http.Client? httpClient,
+  Future<http.Response> Function(http.Request request)? handler,
+  void Function()? onAllEndpointsExhausted,
+}) {
+  return testJellyfinClient(
+    connection: connection ?? testEmbyConnection(),
+    httpClient: httpClient,
+    handler: handler,
+    onAllEndpointsExhausted: onAllEndpointsExhausted,
+  );
+}
+
 PlexClient testPlexClient({
   PlexConfig? config,
   String baseUrl = 'https://plex.example.com',