Serve JPEG XL when a request names no format (closes #222)

A /v1/image/ URL whose last segment is a size with no format, such as
800x600 or orig, is served as JPEG XL and signed as jxl, so it shares
the signature of the same URL ending in .jxl. An encrypted URL whose
token holds no format is served as JPEG XL, as encurl.DefaultFormat is
now jxl. The generator page selects JPEG XL by default, and a form
with an empty format, or none, makes a URL whose name ends in .jxl.

The image processor no longer takes an empty format as orig: both
routes give every request a format, so it refuses a request with none
instead of keeping a second default. auto still ends with JPEG.

Model: opus-5-5

internal/imageprocessor/imageprocessor.go

@@ -256,9 +256,9 @@ func (p *ImageProcessor) Process(
 		}
 	}
 
-	// Determine output format
+	// orig is the source's own format; encode refuses an empty format
 	outputFormat := req.Format
-	if outputFormat == FormatOriginal || outputFormat == "" {
+	if outputFormat == FormatOriginal {
 		outputFormat = p.formatFromString(inputFormat)
 	}