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,

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")]