feat: add dithering param and parallel frame rendering for GIF

- Add dithering bool param to GIFRequest (default true)
- Parallelize frame rendering with goroutines
- Conditional Floyd-Steinberg dithering for speed vs quality tradeoff

internal/api/types.go

@@ -19,6 +19,7 @@ type GIFRequest struct {
 	Width      int             `json:"width"`      // default 512
 	Height     int             `json:"height"`     // default 512
 	Delay      int             `json:"delay"`      // centiseconds between frames, default 5
+	Dithering  *bool           `json:"dithering"`  // Floyd-Steinberg dithering, default true
 }
 
 // ErrorResponse represents an error returned by the API
@@ -56,4 +57,8 @@ func (r *GIFRequest) ApplyDefaults() {
 	if r.Background == "" {
 		r.Background = "#FFFFFF"
 	}
+	if r.Dithering == nil {
+		defaultDithering := true
+		r.Dithering = &defaultDithering
+	}
 }