Files: play video and audio in the viewer, as the file is; reads answer Range requests

The viewer hands a video or audio file to the browser untouched — container, codec and
all — and the browser plays what it can; when it can't, the viewer says so and offers the
download. What the server adds is the part a player needs: /api/files/read now goes
through http.ServeContent for a real file of known size, so Range requests (seeking) and
HEAD work, and a download can resume. The root backend's helper takes an offset for that,
and a Seeker opens the backend once, at the range asked for, instead of streaming from
the top. Audio files are a preview kind too: their cover art is the thumbnail.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

internal/thumbs/thumbs.go

@@ -93,6 +93,7 @@ func (e *DecodeError) Error() string { return e.Msg }
 
 var imageExt = set("png jpg jpeg jpe jfif gif webp bmp tif tiff avif heic heif hif jxl svg ico psd tga thm")
 var videoExt = set("mp4 m4v mkv webm mov avi wmv flv mpg mpeg mpe m2v ts m2ts mts 3gp 3g2 ogv ogm mxf vob asf rm rmvb divx f4v mjpeg mjpg 360 osv insv lrf")
+var audioExt = set("mp3 flac m4a m4b aac ogg oga opus wav wma aiff aif alac ape wv mka")
 
 func set(s string) map[string]bool {
 	m := map[string]bool{}
@@ -102,7 +103,8 @@ func set(s string) map[string]bool {
 	return m
 }
 
-// Kind is "image", "video" or "" for a file name.
+// Kind is "image", "video", "audio" or "" for a file name. An audio file's preview is its
+// cover art, when it carries any.
 func Kind(name string) string {
 	ext := strings.ToLower(strings.TrimPrefix(filepath.Ext(name), "."))
 	switch {
@@ -110,6 +112,8 @@ func Kind(name string) string {
 		return "image"
 	case videoExt[ext]:
 		return "video"
+	case audioExt[ext]:
+		return "audio"
 	}
 	return ""
 }