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-tauri/src/domain/media.rs

@@ -57,6 +57,19 @@ pub enum MediaKind {
     Other,
 }
 
+/// The kind of a media stream within an item (audio track, video track,
+/// subtitle, …) — provider-neutral, replacing the stringly Jellyfin stream type.
+#[derive(specta::Type, Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize, Default)]
+#[serde(rename_all = "camelCase")]
+pub enum StreamKind {
+    Audio,
+    Video,
+    Subtitle,
+    /// Any stream kind we do not model explicitly (e.g. embedded image, data).
+    #[default]
+    Other,
+}
+
 impl MediaKind {
     /// True for kinds that are containers/collections rather than playable leaves.
     /// Presentation-neutral helper the backend can use for e.g. drill-vs-play.