fix(media): reject extension cookie headers for media

Keep explicit media requests on yt-dlp's configured browser-cookie path and preserve cookies for normal captures.

src/store/useDownloadStore.test.ts

@@ -818,6 +818,7 @@ describe('useDownloadStore', () => {
     expect(state.isAddModalOpen).toBe(true);
     expect(state.pendingAddUrls).toBe('https://adult.example/watch/123');
     expect(state.pendingAddMediaUrls).toEqual(['https://adult.example/watch/123']);
+    expect(state.pendingAddCookies).toBe('');
   });
 
   it('deduplicates forced media URLs and drops stale media intent when opening fresh', async () => {