Add WebP encoding support

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

internal/imgcache/processor.go

@@ -12,6 +12,7 @@ import (
 	"io"
 
 	"github.com/disintegration/imaging"
+	gowebp "github.com/gen2brain/webp"
 	"golang.org/x/image/webp"
 )
 
@@ -122,6 +123,7 @@ func (p *ImageProcessor) SupportedOutputFormats() []ImageFormat {
 		FormatJPEG,
 		FormatPNG,
 		FormatGIF,
+		FormatWebP,
 	}
 }
 
@@ -227,7 +229,14 @@ func (p *ImageProcessor) encode(img image.Image, format ImageFormat, quality int
 		}
 
 	case FormatWebP:
-		return nil, fmt.Errorf("%w: WebP encoding not supported", ErrUnsupportedOutputFormat)
+		options := gowebp.Options{
+			Lossless: false,
+			Quality:  quality,
+		}
+		err := gowebp.Encode(&buf, img, options)
+		if err != nil {
+			return nil, err
+		}
 
 	case FormatAVIF:
 		return nil, fmt.Errorf("%w: AVIF encoding not supported", ErrUnsupportedOutputFormat)

internal/templates/generator.html

@@ -108,6 +108,7 @@
                         <option value="orig" {{if eq .FormFormat "orig"}}selected{{end}}>Original</option>
                         <option value="jpeg" {{if eq .FormFormat "jpeg"}}selected{{end}}>JPEG</option>
                         <option value="png" {{if eq .FormFormat "png"}}selected{{end}}>PNG</option>
+                        <option value="webp" {{if eq .FormFormat "webp"}}selected{{end}}>WebP</option>
                         <option value="gif" {{if eq .FormFormat "gif"}}selected{{end}}>GIF</option>
                     </select>
                 </div>