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/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,