First startup fix

[66OJ66]

Hi,

When starting shadplay for the 1st time, I got this error Error: No such file or directory (os error 2).

After doing some digging, it looks like UserConfig isn't created automatically.

So this PR simply:

• Uses the default UserConfig when no existing config is found (and saves this to disk)
• Removes the use of anyhow from fn main as config loading always succeeds now