Serve the format auto, chosen from the Accept header (closes #88)

auto is a format in the /v1/image/ path, an encrypted URL's token and
the generator page. Once the signature or token is checked, pixa
chooses AVIF when Accept names image/avif, else WebP when it names
image/webp, else JPEG when the most specific of image/jpeg, image/* and
*/* allows it or Accept is absent. q=0 refuses a format; a header
allowing none of the three answers 406, one that does not parse 400.
The signature and token cover auto itself; the cache key and ETag use
the chosen format. Answers from then on carry Vary: Accept.

Model: opus-5-5

internal/templates/generator.html

@@ -95,6 +95,7 @@
                     </label>
                     <select id="format" name="format">
                         <option value="orig" {{if eq .FormFormat "orig"}}selected{{end}}>Original</option>
+                        <option value="auto" {{if eq .FormFormat "auto"}}selected{{end}}>Auto (AVIF, WebP or JPEG)</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>