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/i18n/bg.i18n.json

@@ -11,7 +11,7 @@
     "waitingForAuth": "Изчакване на удостоверяване...\nВлезте от браузъра си.",
     "useBrowser": "Използвай браузър",
     "or": "или",
-    "connectToJellyfin": "Свържи се с Jellyfin",
+    "connectToMediaBrowser": "",
     "useQuickConnect": "Използвай Quick Connect",
     "quickConnectInstructions": "Отворете Quick Connect в Jellyfin и въведете този код.",
     "quickConnectWaiting": "Изчакване на одобрение…",
@@ -827,8 +827,8 @@
     "sessionExpiredOne": "Сесията за ${name} е изтекла",
     "sessionExpiredMany": "Сесиите за ${count} сървъра са изтекли",
     "signInAgain": "Влез отново",
-    "editJellyfinTitle": "Редактиране на Jellyfin връзка",
-    "editJellyfinIntro": "Добавете или премахнете URL адреси за ${serverName}. Plezy ще използва достъпния URL адрес с най-ниска латентност."
+    "editMediaBrowserTitle": "",
+    "editMediaBrowserIntro": ""
   },
   "discover": {
     "title": "Открий",
@@ -1873,12 +1873,12 @@
     }
   },
   "addServer": {
-    "addJellyfinTitle": "Добави Jellyfin сървър",
+    "addMediaBrowserTitle": "",
     "serverUrls": "URL адреси на сървъра",
     "serverUrlsHelper": "Позволени са няколко URL адреса, разделени със запетаи.",
     "findServer": "Намери сървър",
-    "searchingLocalServers": "Търсене на локални Jellyfin сървъри...",
-    "localServers": "Локални Jellyfin сървъри",
+    "searchingLocalMediaBrowserServers": "",
+    "localMediaBrowserServers": "",
     "username": "Потребителско име",
     "password": "Парола",
     "signIn": "Вход",
@@ -1890,15 +1890,15 @@
     "addPlexTitle": "Вход с Plex",
     "pinExpired": "PIN-ът изтече преди вход. Моля, опитайте отново.",
     "failedToRegisterAccount": "Неуспешна регистрация на акаунт: ${error}",
-    "enterJellyfinUrlError": "Въведете URL адреса на вашия Jellyfin сървър",
+    "enterMediaBrowserUrlError": "",
     "addConnectionTitle": "Добави връзка",
     "addConnectionTitleScoped": "Добави към ${name}",
     "signInWithPlexCard": "Вход с Plex",
     "signInWithPlexCardSubtitle": "Удостоверете това устройство. Споделените сървъри се добавят.",
     "signInWithPlexCardSubtitleScoped": "Удостоверете Plex акаунт. Домашните потребители стават профили.",
-    "connectToJellyfinCard": "Свързване с Jellyfin",
-    "connectToJellyfinCardSubtitle": "Въведете URL адрес на сървъра, потребителско име и парола.",
-    "connectToJellyfinCardSubtitleScoped": "Вход в Jellyfin сървър. Свързва се с ${name}.",
+    "connectToMediaBrowserCard": "",
+    "connectToMediaBrowserCardSubtitle": "",
+    "connectToMediaBrowserCardSubtitleScoped": "",
     "borrowFromAnotherProfile": "Използвай от друг профил",
     "borrowFromAnotherProfileSubtitle": "Използвай връзка от друг профил. PIN-защитените профили изискват PIN."
   }