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/widgets/backend_badge.dart

@@ -3,8 +3,8 @@ import 'package:flutter_svg/flutter_svg.dart';
 
 import '../media/media_backend.dart';
 
-/// Tiny SVG badge for a [MediaBackend] (Plex chevron / Jellyfin mark).
-/// Both assets render in `currentColor` so they pick up whatever foreground
+/// Tiny SVG badge for a [MediaBackend] (Plex chevron / Jellyfin or Emby mark).
+/// All assets render in `currentColor` so they pick up whatever foreground
 /// the parent provides — pass [color] to override, otherwise inherits from
 /// [DefaultTextStyle] / `IconTheme`.
 class BackendBadge extends StatelessWidget {
@@ -24,6 +24,7 @@ class BackendBadge extends StatelessWidget {
     final asset = switch (backend) {
       MediaBackend.plex => 'assets/plex_chevron.svg',
       MediaBackend.jellyfin => 'assets/jellyfin_icon.svg',
+      MediaBackend.emby => 'assets/emby_icon.svg',
     };
     return SvgPicture.asset(
       asset,

lib/widgets/library_management_sheet.dart

@@ -99,9 +99,9 @@ Future<void> showLibraryManagementSheet(
 }
 
 List<ContextMenuItem> _getLibraryMenuItems(MediaLibrary library) {
-  // Refresh metadata is the only admin action both backends support — Plex
-  // hits `/library/sections/{id}/refresh?force=1`, Jellyfin posts to
-  // `/Items/{id}/Refresh` (the library view is itself an item).
+  // Refresh metadata is the only admin action every backend supports — Plex
+  // hits `/library/sections/{id}/refresh?force=1`; MediaBrowser servers post
+  // to `/Items/{id}/Refresh` (the library view is itself an item).
   final refresh = ContextMenuItem(
     value: 'refresh',
     icon: Symbols.sync_rounded,
@@ -112,7 +112,7 @@ List<ContextMenuItem> _getLibraryMenuItems(MediaLibrary library) {
     isDestructive: true,
   );
   // Scan / analyze / empty trash hit Plex-only endpoints, so backend
-  // capability gating keeps them out of Jellyfin menus. The library-qualified
+  // capability gating keeps them out of MediaBrowser menus. The library-qualified
   // resolver independently requires the exact owning Plex server.
   if (library.backend != MediaBackend.plex) return [refresh];
   return [

lib/widgets/media_context_menu.dart

@@ -84,13 +84,13 @@ bool isAdminActionAllowedForMediaItem({
 
 /// Whether "Delete from server" may be offered for an item.
 ///
-/// Deliberately not folded into [isAdminActionAllowedForMediaItem]: on Jellyfin
+/// Deliberately not folded into [isAdminActionAllowedForMediaItem]: on MediaBrowser servers
 /// the admin bit says nothing about deletion. `BaseItem.IsAuthorizedToDelete`
 /// consults `EnableContentDeletion` and the per-library grant only, and only
 /// the auto-created first user gets the former for free — so an administrator
 /// can lack the right (issue #1749) and a plain user can hold it. The server's
 /// per-item answer ([resolvedItemPermission], from
-/// [MediaDeletionPermissionClient]) is therefore the sole Jellyfin condition,
+/// [MediaDeletionPermissionClient]) is therefore the sole MediaBrowser condition,
 /// and anything unknown — offline, request failed, timed out, item invisible —
 /// stays hidden rather than offering a button that 401s.
 ///
@@ -102,7 +102,7 @@ bool isMediaDeletionAllowed({
   required bool isAdminActionAllowed,
 }) => switch (itemBackend) {
   null => false,
-  MediaBackend.jellyfin => resolvedItemPermission == true,
+  MediaBackend.jellyfin || MediaBackend.emby => resolvedItemPermission == true,
   MediaBackend.plex => isAdminActionAllowed,
 };
 
@@ -274,7 +274,7 @@ class MediaContextMenuState extends State<MediaContextMenu> {
     final isCollection = mediaKind == MediaKind.collection;
 
     // Backend-aware gate: a few menu items remain Plex-only because the
-    // server-side feature has no Jellyfin equivalent (match/unmatch).
+    // server-side feature has no MediaBrowser equivalent (match/unmatch).
     // No fallback: items without a backend marker show only neutral actions —
     // dispatching a Plex-only action against an unknown-backend item could
     // crash or hit the wrong server.
@@ -290,7 +290,7 @@ class MediaContextMenuState extends State<MediaContextMenu> {
 
     // Check if user has admin privileges. Backend-neutral: Plex uses the
     // server-owned flag (folded with the active Plex Home profile's admin
-    // bit, when applicable); Jellyfin uses `JellyfinConnection.isAdministrator`
+    // bit, when applicable); MediaBrowser servers use `JellyfinConnection.isAdministrator`
     // captured at sign-in.
     final multiServerProvider = Provider.of<MultiServerProvider>(context, listen: false);
     final activeProfile = context.read<ActiveProfileProvider>().active;
@@ -476,7 +476,7 @@ class MediaContextMenuState extends State<MediaContextMenu> {
         );
       }
 
-      // Match / Unmatch — Plex-only (Jellyfin doesn't expose match agents).
+      // Match / Unmatch — Plex-only (MediaBrowser servers don't expose match agents).
       if (isPlex && isAdmin && (mediaKind == MediaKind.movie || mediaKind == MediaKind.show)) {
         final isUnmatched = _isUnmatched(mediaItem);
         menuActions.add(
@@ -492,8 +492,8 @@ class MediaContextMenuState extends State<MediaContextMenu> {
       }
 
       // Remove from Collection (only when viewing items within a collection).
-      // Plex-only — uses `removeFromCollection` API; Jellyfin's collection
-      // membership API isn't wired here yet.
+      // Plex-only — uses `removeFromCollection` API; MediaBrowser collection
+      // membership APIs aren't wired here yet.
       if (isPlex && widget.collectionId != null) {
         menuActions.add(
           _MenuAction(
@@ -620,7 +620,7 @@ class MediaContextMenuState extends State<MediaContextMenu> {
 
       // Add to... (for episodes, movies, shows, and seasons). Plex-only —
       // uses `buildMetadataUri` + `addToPlaylist` / `addToCollection`. The
-      // Jellyfin item-add API is different and not wired here yet.
+      // MediaBrowser item-add APIs are different and not wired here yet.
       if (isPlex &&
           (mediaKind == MediaKind.episode ||
               mediaKind == MediaKind.movie ||
@@ -630,9 +630,9 @@ class MediaContextMenuState extends State<MediaContextMenu> {
       }
 
       // Delete media item (for episodes, movies, shows, and seasons). Routed
-      // through `MediaServerClient.deleteMediaItem`, which both Plex and
-      // Jellyfin implement (DELETE /library/metadata/{id} and
-      // DELETE /Items/{id} respectively); the kind and permission checks were
+      // through `MediaServerClient.deleteMediaItem`, which every backend
+      // implements (DELETE /library/metadata/{id} for Plex and
+      // DELETE /Items/{id} for MediaBrowser servers); the kind and permission checks were
       // resolved together above.
       if (canDeleteFromServer) {
         menuActions.add(

lib/widgets/rating_bottom_sheet.dart

@@ -568,6 +568,7 @@ class _RatingBottomSheetState extends State<RatingBottomSheet> {
   String _backendLabel(MediaBackend backend) => switch (backend) {
     MediaBackend.plex => 'Plex',
     MediaBackend.jellyfin => 'Jellyfin',
+    MediaBackend.emby => 'Emby',
   };
 }