test(60): prove the failed-flag reset effect actually runs on channel switch

The reset moved from render-phase into an effect; the existing switch test
asserted only that the error banner cleared, which stays green even if the
effect is deleted. Now the new channel must also reach 'playing'.

Negative control: disabling the reset fails exactly this test, and only it.

Refs #60

web/src/screens/channels/ChannelPreviewPanel.test.tsx

@@ -287,6 +287,17 @@ describe('ChannelPreviewPanel', () => {
 
       expect(screen.queryByText(/networkError: manifestLoadError/)).not.toBeInTheDocument();
       expect(hlsMock.loadSource).toHaveBeenLastCalledWith('/iptv/channel/34.1.m3u8');
+
+      // The failed-flag reset runs in an effect, not during render, so prove it actually ran: the
+      // new channel must still be able to reach `playing`. Without this assertion, deleting that
+      // reset leaves the suite green while the new channel sits at `starting` forever, because
+      // onPlaying would keep returning early on the PREVIOUS channel's stale failed flag.
+      const video = document.body.querySelector('video');
+      act(() => {
+        video?.dispatchEvent(new Event('playing'));
+      });
+
+      expect(screen.getByText('playing')).toBeInTheDocument();
     });
 
     it('does not carry the retry play token over to the next channel', () => {