domain: neutral StreamKind for media streams (phase 4d)

Add StreamKind enum (audio/video/subtitle/other) to the domain module with
a total stream_kind_from_jellyfin mapper. MediaStream gains a kind field
(dual-carry), populated at the mapping seam. Frontend VideoPlayer track/
subtitle selection and the channel-video check now use stream.kind instead
of the Jellyfin stream.type string.

Rust 456 (+ stream_kinds_map test), frontend 644, check clean.

src/lib/api/bindings.ts

@@ -1912,7 +1912,16 @@ export type MediaSource = { id: string; name: string; container?: string | null;
 /**
  * Media stream information (audio, video, subtitle tracks)
  */
-export type MediaStream = { type: string; codec?: string | null; language?: string | null; displayTitle?: string | null; index: number; isDefault: boolean; isForced: boolean }
+export type MediaStream = { 
+/**
+ * Legacy Jellyfin stream type string ("Audio"/"Video"/"Subtitle"). Being
+ * replaced by `kind`; dual-carried while the frontend migrates.
+ */
+type: string; 
+/**
+ * Provider-neutral stream classification — replaces `stream_type`.
+ */
+kind?: StreamKind; codec?: string | null; language?: string | null; displayTitle?: string | null; index: number; isDefault: boolean; isForced: boolean }
 export type MediaType = "audio" | "video"
 /**
  * Lightweight media item for merged playback state
@@ -2516,6 +2525,15 @@ export type SmartCacheStats = { total_size: number; storage_limit: number; avail
  * Storage statistics for downloads
  */
 export type StorageStats = { total_bytes: number; total_items: number; albums: AlbumStorageInfo[] }
+/**
+ * The kind of a media stream within an item (audio track, video track,
+ * subtitle, …) — provider-neutral, replacing the stringly Jellyfin stream type.
+ */
+export type StreamKind = "audio" | "video" | "subtitle" | 
+/**
+ * Any stream kind we do not model explicitly (e.g. embedded image, data).
+ */
+"other"
 /**
  * Represents a subtitle track
  */