Fix method redefined warning for support_email

[joemasilotti]

Pull Request

Summary:
Fix Ruby's "method redefined" warning.

Related Issue:
N/A

Description:
mattr_accessor :support_email creates a support_email= writer, but we immediately redefine it to add Mail::Address casting. This triggers Ruby's "method redefined" warning when running with warnings enabled.

Use mattr_reader instead since we define the writer manually.

Testing:

# Before (with mattr_accessor)
$ ruby -w -e "require 'pay'"
warning: method redefined; discarding old support_email=

# After (with mattr_reader)
$ ruby -w -e "require 'pay'"
# No warning

Checklist:

• Code follows the project's coding standards
• Tests have been added or updated to cover the changes
• Documentation has been updated (if applicable)
• All existing tests pass
• Conforms to the contributing guidelines

Additional Notes:
One-line change. No functional difference, support_email getter and setter work identically.