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/from_jellyfin.rs

@@ -6,7 +6,18 @@
 //!
 //! Spec: docs/specs/frontend-domain-model.md
 
-use super::media::MediaKind;
+use super::media::{MediaKind, StreamKind};
+
+/// Classify a Jellyfin media-stream `Type` string into a neutral [`StreamKind`].
+/// Total and panic-free.
+pub fn stream_kind_from_jellyfin(stream_type: &str) -> StreamKind {
+    match stream_type {
+        "Audio" => StreamKind::Audio,
+        "Video" => StreamKind::Video,
+        "Subtitle" => StreamKind::Subtitle,
+        _ => StreamKind::Other,
+    }
+}
 
 /// Jellyfin ticks per second (10 million). A tick is 100 ns.
 /// The frontend must never see ticks — this is where they die.
@@ -143,6 +154,18 @@ mod tests {
         );
     }
 
+    #[test]
+    fn stream_kinds_map() {
+        assert_eq!(stream_kind_from_jellyfin("Audio"), StreamKind::Audio);
+        assert_eq!(stream_kind_from_jellyfin("Video"), StreamKind::Video);
+        assert_eq!(stream_kind_from_jellyfin("Subtitle"), StreamKind::Subtitle);
+        assert_eq!(
+            stream_kind_from_jellyfin("EmbeddedImage"),
+            StreamKind::Other
+        );
+        assert_eq!(stream_kind_from_jellyfin(""), StreamKind::Other);
+    }
+
     #[test]
     fn unknown_type_never_panics_and_falls_back() {
         // The whole point: garbage in, safe kind out, no panic.

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.

src-tauri/src/domain/mod.rs

@@ -6,5 +6,5 @@
 pub mod from_jellyfin;
 pub mod media;
 
-pub use from_jellyfin::{kind_from_jellyfin, ticks_to_ms};
-pub use media::MediaKind;
+pub use from_jellyfin::{kind_from_jellyfin, stream_kind_from_jellyfin, ticks_to_ms};
+pub use media::{MediaKind, StreamKind};

src-tauri/src/repository/online.rs

@@ -658,6 +658,7 @@ impl JellyfinItem {
                 streams
                     .into_iter()
                     .map(|s| crate::repository::types::MediaStream {
+                        kind: crate::domain::stream_kind_from_jellyfin(&s.stream_type),
                         stream_type: s.stream_type,
                         codec: s.codec,
                         language: s.language,

src-tauri/src/repository/types.rs

@@ -205,8 +205,13 @@ pub struct MediaItem {
 #[derive(specta::Type, Debug, Clone, Serialize, Deserialize)]
 #[serde(rename_all = "camelCase")]
 pub struct MediaStream {
+    /// Legacy Jellyfin stream type string ("Audio"/"Video"/"Subtitle"). Being
+    /// replaced by `kind`; dual-carried while the frontend migrates.
     #[serde(rename = "type")]
     pub stream_type: String,
+    /// Provider-neutral stream classification — replaces `stream_type`.
+    #[serde(default)]
+    pub kind: crate::domain::StreamKind,
     #[serde(skip_serializing_if = "Option::is_none")]
     pub codec: Option<String>,
     #[serde(skip_serializing_if = "Option::is_none")]