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.