Fix empty download links by using lecture media API

[basita512]

Summary

• Switch from the obsolete /assets/{id}?stream_urls endpoint to the subscribed-courses lecture API and parse media_sources / download_urls / stream_urls.
• Show available resolutions on mismatch and continue to the next lecture instead of hard-exiting after failed retries.
• Add Go module files and rebuild the Windows binary.

Test plan

• Authenticate with a valid Udemy access token
• Select a subscribed course and lecture range
• Confirm available resolutions are listed when an invalid one is entered
• Confirm downloads succeed for a non-DRM lecture at a listed resolution
• Confirm DRM-only lectures are skipped with a clear message instead of crashing