Fix brand account support by adding X-Goog-AuthUser header

[hamza-tm]

Hi, found an issue when using a brand account where no playlists would show up. Removing the account_id (i.e. going back to the main account) worked fine however.

After some digging it turns out we're missing a header. This PR just adds support for passing that header through. Have tested this locally and brand accounts work correctly for me now.

Summary

• Fixes brand account playlists not showing up by adding the required X-Goog-AuthUser header to API requests
• Adds auth_user field to YoutubeMusicInstance struct
• Reads X-Goog-AuthUser from headers.txt in from_header_file()
• Includes header in browse_raw and browse_continuation_raw API requests
• Updates README with documentation for brand account setup

Test plan

• Tested with brand account - playlists now load correctly
• Verified backward compatibility - works without X-Goog-AuthUser header for non-brand accounts

Fixes #106

🤖 Generated with Claude Code

[ccgauche]

Thanks for the contribution. Just why aren't you just propagating the headers through the HeadMap further ?

[ccgauche]

Here you are parsing the file again after having skipped the x-ggog-authuser but in reality you can just add a match case next to cookie and user-agent for it to be propagated.

[ccgauche]

I was pretty sure it was also supported and it is:

(Optional) Using a brand account

    If you use a second account for youtube music
        Go to https://myaccount.google.com/
        Switch to your brand account
        copy the number written in the url, after \b\
        paste it into a new account_id.txt file in the same folder as headers.txt
``

[restalex]

I always used my branded account and I did not have any issue apart from refreshing the cookies sometimes.
I've tried this patch. In my case, nothing changes with or without the X-Goog-AuthUser as expected.
Maybe it is because my X-Goog-AuthUser. is 0. What its your value 0 or 1?
Have you tried without the X-Goog-AuthUser but with refreshed cookies?