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/connection/connection.dart

@@ -1,4 +1,5 @@
 import '../media/media_backend.dart';
+import '../media/media_browser_dialect.dart';
 import '../models/plex/plex_home_user.dart';
 import '../services/plex_auth_service.dart';
 import '../utils/json_utils.dart';
@@ -9,22 +10,38 @@ import '../utils/url_utils.dart';
 /// (e.g. database column values).
 enum ConnectionKind {
   plex,
-  jellyfin;
+  jellyfin,
+  emby;
 
   String get id => switch (this) {
     ConnectionKind.plex => 'plex',
     ConnectionKind.jellyfin => 'jellyfin',
+    ConnectionKind.emby => 'emby',
   };
 
   static ConnectionKind fromId(String id) => switch (id) {
     'plex' => ConnectionKind.plex,
     'jellyfin' => ConnectionKind.jellyfin,
+    'emby' => ConnectionKind.emby,
     _ => throw ArgumentError('Unknown ConnectionKind id: $id'),
   };
 
   MediaBackend get backend => switch (this) {
     ConnectionKind.plex => MediaBackend.plex,
     ConnectionKind.jellyfin => MediaBackend.jellyfin,
+    ConnectionKind.emby => MediaBackend.emby,
+  };
+
+  /// The MediaBrowser dialect this kind speaks, or `null` for Plex.
+  MediaBrowserDialect? get dialect => switch (this) {
+    ConnectionKind.plex => null,
+    ConnectionKind.jellyfin => MediaBrowserDialect.jellyfin,
+    ConnectionKind.emby => MediaBrowserDialect.emby,
+  };
+
+  static ConnectionKind fromDialect(MediaBrowserDialect dialect) => switch (dialect) {
+    MediaBrowserDialect.jellyfin => ConnectionKind.jellyfin,
+    MediaBrowserDialect.emby => ConnectionKind.emby,
   };
 }
 
@@ -188,7 +205,9 @@ class PlexAccountConnection extends Connection {
   }
 }
 
-/// A single-server Jellyfin connection.
+/// A single-server connection to a MediaBrowser-family server — Jellyfin or its
+/// Emby ancestor. [dialect] selects which of the two wire dialects this
+/// connection speaks; every other field has the same meaning on both.
 class JellyfinConnection extends Connection {
   @override
   final String id;
@@ -202,10 +221,14 @@ class JellyfinConnection extends Connection {
   @override
   final DateTime? lastAuthenticatedAt;
 
+  /// Which MediaBrowser dialect this server speaks. Drives [kind], [backend]
+  /// and every route/capability delta in [JellyfinClient].
+  final MediaBrowserDialect dialect;
+
   /// Active server base URL, no trailing slash. e.g. `https://jellyfin.home.lan`.
   final String baseUrl;
 
-  /// Candidate server URLs for this Jellyfin server, with [baseUrl] first.
+  /// Candidate server URLs for this server, with [baseUrl] first.
   /// Existing installs only have [baseUrl]; deserialization backfills this.
   final List<String> baseUrls;
 
@@ -215,7 +238,7 @@ class JellyfinConnection extends Connection {
   /// Server's machine identifier (System/Info `Id`).
   final String serverMachineId;
 
-  /// Authenticated Jellyfin user id (UUID).
+  /// Authenticated user id. A UUID on Jellyfin, an opaque hex string on Emby.
   final String userId;
 
   /// Authenticated user's display name.
@@ -228,13 +251,13 @@ class JellyfinConnection extends Connection {
   /// `Authorization: MediaBrowser DeviceId="..."` header).
   final String deviceId;
 
-  /// Whether this user is a Jellyfin admin (`/Users/{id}.Policy.IsAdministrator`).
+  /// Whether this user is a server admin (`/Users/{id}.Policy.IsAdministrator`).
   /// Captured at auth time so the UI can gate admin-only entries (delete,
   /// match/unmatch, edit metadata) without an extra round-trip.
   final bool isAdministrator;
 
   /// The authenticated user's `PrimaryImageTag`, or `null` when they have no
-  /// profile picture. Jellyfin omits the key entirely in that case, and the
+  /// profile picture. The server omits the key entirely in that case, and the
   /// tag is `MD5(imagePath + lastModified)` so it changes on every upload —
   /// which makes the derived avatar URL self-invalidating. Captured at auth
   /// time and refreshed by [JellyfinClient.checkHealth].
@@ -250,6 +273,7 @@ class JellyfinConnection extends Connection {
     required this.userName,
     required this.accessToken,
     required this.deviceId,
+    this.dialect = MediaBrowserDialect.jellyfin,
     this.isAdministrator = false,
     this.primaryImageTag,
     this.status = ConnectionStatus.unknown,
@@ -259,7 +283,7 @@ class JellyfinConnection extends Connection {
        baseUrls = _normalizeBaseUrls(baseUrl, baseUrls);
 
   @override
-  ConnectionKind get kind => ConnectionKind.jellyfin;
+  ConnectionKind get kind => ConnectionKind.fromDialect(dialect);
 
   @override
   String get displayName => '$userName · $serverName';
@@ -306,11 +330,12 @@ class JellyfinConnection extends Connection {
     String? userName,
     String? accessToken,
     String? deviceId,
+    MediaBrowserDialect? dialect,
     bool? isAdministrator,
     String? primaryImageTag,
 
-    /// Deleting a Jellyfin profile picture drops `PrimaryImageTag` from the
-    /// user DTO, so a refresh must be able to null the cached value — a bare
+    /// Deleting a profile picture drops `PrimaryImageTag` from the user DTO, so
+    /// a refresh must be able to null the cached value — a bare
     /// `primaryImageTag: null` is indistinguishable from "unchanged".
     bool clearPrimaryImageTag = false,
     ConnectionStatus? status,
@@ -328,6 +353,7 @@ class JellyfinConnection extends Connection {
       userName: userName ?? this.userName,
       accessToken: accessToken ?? this.accessToken,
       deviceId: deviceId ?? this.deviceId,
+      dialect: dialect ?? this.dialect,
       isAdministrator: isAdministrator ?? this.isAdministrator,
       primaryImageTag: clearPrimaryImageTag ? null : (primaryImageTag ?? this.primaryImageTag),
       status: status ?? this.status,
@@ -336,6 +362,9 @@ class JellyfinConnection extends Connection {
     );
   }
 
+  /// The persisted payload deliberately omits [dialect]: the `connections.kind`
+  /// column is the authoritative, indexed discriminator and
+  /// [JellyfinConnection.fromConfigJson] receives it from there.
   @override
   Map<String, Object?> toConfigJson() {
     return {
@@ -358,6 +387,7 @@ class JellyfinConnection extends Connection {
     required ConnectionStatus status,
     required DateTime createdAt,
     DateTime? lastAuthenticatedAt,
+    MediaBrowserDialect dialect = MediaBrowserDialect.jellyfin,
   }) {
     final rawBaseUrls = json['baseUrls'];
     final baseUrls = rawBaseUrls is List ? rawBaseUrls.whereType<String>().toList(growable: false) : const <String>[];
@@ -369,7 +399,8 @@ class JellyfinConnection extends Connection {
       id: id,
       baseUrl: baseUrl,
       baseUrls: baseUrls,
-      serverName: json['serverName'] as String? ?? 'Jellyfin',
+      dialect: dialect,
+      serverName: json['serverName'] as String? ?? dialect.productName,
       serverMachineId: json['serverMachineId'] as String? ?? '',
       userId: json['userId'] as String? ?? '',
       userName: json['userName'] as String? ?? '',

lib/connection/connection_registry.dart

@@ -161,12 +161,13 @@ class ConnectionRegistry {
           createdAt: createdAt,
           lastAuthenticatedAt: lastAuth,
         ),
-        ConnectionKind.jellyfin => JellyfinConnection.fromConfigJson(
+        ConnectionKind.jellyfin || ConnectionKind.emby => JellyfinConnection.fromConfigJson(
           id: row.id,
           json: revealed.config,
           status: ConnectionStatus.unknown,
           createdAt: createdAt,
           lastAuthenticatedAt: lastAuth,
+          dialect: kind.dialect!,
         ),
       };
       if (revealed.migrated) {