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/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>