When WebRTC is disabled, no visible warning is shown in Firefox

If WebRTC is disabled in the Firefox, FilePizza silently fails with "The current browser does not support WebRTC" in the console, but shows no UI warning. As a result, the user sees "Initializing WebRTC peer..." indefinitely with no feedback. Suggest detecting disabled WebRTC and showing a visible alert or fallback message.

In my case, I had media.peerconnection.enabled = false, and there was no indication on the page that WebRTC was blocked. I only realized the problem by checking the browser console. Once I enabled WebRTC support (media.peerconnection.enabled = true), everything started working immediately.

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