[FEATURE] Add AVIF support in smart file size optimization options

[csarigoz]

Since AVIF is a popular, high compression file type, we'll add it to the smart file size optimization options.

[csarigoz]

🎩 AVIF support has been implemented in PR #4. The implementation:

• Adds AVIF to smart format selection alongside JPEG, PNG, and WebP
• Uses pillow-avif-plugin for encoding (gracefully optional)
• Automatically picks the smallest output format

Ready for review!