Uses github.com/gen2brain/webp - a CGO-free library that uses WASM via
wazero runtime for encoding. WebP decoding was already supported.
- Add gen2brain/webp dependency for encoding
- Implement WebP encoding in processor.go
- Add FormatWebP to SupportedOutputFormats
- Re-enable WebP option in generator form dropdown
- Mark WebP encoding as complete in TODO.md