Add reverse file with otpauth://... URLs to migration QR codes

[idachev]

Recently, I made a stupid mistake by exporting my Google Authenticator QR codes, but I was quick and did not read the last screen, which was to remove all OTPs from the app.

I lost the exported screens but fortunately have backups of my otpauth URLs from Linux OTPClient.

So what I add with this pull request is to grab a file exported from OTPClient, which li list of otpauth://... URLs and make otpauth-migration... QR images so I can back import these in my Google Authenticator.

Know that this code was generated and enhanced with claude code as I have 25 years of software experience but never touched the go language world until now :)

[dim13]

Thank you for your contribution. However I'm not able to accept it as-is and will need to rework it from ground up first. The quality of AI generated code (here in particular) is on pair with 5-years-old writing his first Basic program.