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/lib/components/player/VideoPlayer.svelte

@@ -180,7 +180,7 @@
       console.log("[VideoPlayer] No media or mediaStreams available");
       return [];
     }
-    const tracks = media.mediaStreams.filter(stream => stream.type === "Audio");
+    const tracks = media.mediaStreams.filter(stream => stream.kind === "audio");
     console.log("[VideoPlayer] Found audio tracks:", tracks.length, tracks);
     return tracks;
   });
@@ -243,7 +243,7 @@
       console.log("[VideoPlayer] No media or mediaStreams available for subtitles");
       return [];
     }
-    const tracks = media.mediaStreams.filter(stream => stream.type === "Subtitle");
+    const tracks = media.mediaStreams.filter(stream => stream.kind === "subtitle");
     console.log("[VideoPlayer] Found subtitle tracks:", tracks.length, tracks);
     return tracks;
   });
@@ -520,7 +520,7 @@
         // Build subtitle tracks for native player
         const subtitleTracks = [];
         if (media.mediaStreams && mediaSourceId) {
-          const subtitles = media.mediaStreams.filter(s => s.type === "Subtitle");
+          const subtitles = media.mediaStreams.filter(s => s.kind === "subtitle");
           for (const sub of subtitles) {
             try {
               const url = await getSubtitleUrl(sub.index);