Fix upload_code limit message

[r4victor]

The error message reported size in base 10 (MB) while the default limit is in base 2 and dstack usually reports sizes in base 2 (MiB):

Repo diff size is 3.2 MB, which exceeds the limit of 2.1 MB. Use .gitignore to 
exclude large files from the repo. This limit can be modified by setting the 
DSTACK_SERVER_CODE_UPLOAD_LIMIT environment variable

Also the error message could be unclear due to rounding:

Repo diff size is 2.1 MB, which exceeds the limit of 2.1 MB. Use .gitignore to 
exclude large files from the repo. This limit can be modified by setting the 
DSTACK_SERVER_CODE_UPLOAD_LIMIT environment variable

Fixed by always displaying size in base 2 and also displaying bytes for close sizes:

Repo diff size is 3.0MiB, which exceeds the limit of 2.0MiB. Use .gitignore to 
exclude large files from the repo. This limit can be modified by setting the 
DSTACK_SERVER_CODE_UPLOAD_LIMIT environment variable.

Repo diff size is 3183563B, which exceeds the limit of 3145728B. Use .gitignore 
to exclude large files from the repo. This limit can be modified by setting the 
DSTACK_SERVER_CODE_UPLOAD_LIMIT environment variable.

Also dropped humanize dependency in favor of existing function sizeof_fmt in #2692.