fix(playback): escape commas in mpv http header fields (#1321)

.changes/stalker-mpv-header-comma-escape.md

@@ -0,0 +1,10 @@
+---
+type: fix
+area: playback
+---
+
+Stalker live TV plays again in the embedded MPV player. The MAG250 user agent
+contains a comma (`KHTML, like Gecko`), which MPV splits in its comma-separated
+`--http-header-fields` list, truncating `X-User-Agent` so strict portals reject
+the stream with HTTP 400. Commas and backslashes in header values are now
+escaped on the Windows/Linux and macOS embedded-MPV paths.