fix: use playback selector in MediaPanel

apps/web/src/__tests__/prefer-derived.test.ts

@@ -9,10 +9,10 @@ test("prefer mp4 derived over original", () => {
   expect(picked).toEqual({ kind: "video_mp4", size: 720 });
 });
 
-test("falls back to original when no derived", () => {
+test("returns null when no mp4 variants", () => {
   const picked = pickVideoPlaybackVariant({
     originalMimeType: "video/x-matroska",
     variants: [],
   });
-  expect(picked).toEqual({ kind: "original" });
+  expect(picked).toBeNull();
 });