fix(server): use http.TimeFormat for Last-Modified header (#5219)

Navidrome returns Last-Modified values like `Fri, 12 Dec 2025 03:32:26
UTC`. This is invalid according to RFC 7231 which requires HTTP dates to
use GMT instead of UTC. Switch to http.TimeFormat instead of
time.RFC1123 to resolve the issue.

server/public/handle_images.go

@@ -60,7 +60,7 @@ func (pub *Router) handleImages(w http.ResponseWriter, r *http.Request) {
 
 	defer imgReader.Close()
 	w.Header().Set("Cache-Control", "public, max-age=315360000")
-	w.Header().Set("Last-Modified", lastUpdate.Format(time.RFC1123))
+	w.Header().Set("Last-Modified", lastUpdate.Format(http.TimeFormat))
 	cnt, err := io.Copy(w, imgReader)
 	if err != nil {
 		log.Warn(ctx, "Error sending image", "count", cnt, err)

server/subsonic/media_retrieval.go

@@ -81,7 +81,7 @@ func (api *Router) GetCoverArt(w http.ResponseWriter, r *http.Request) (*respons
 
 	defer imgReader.Close()
 	w.Header().Set("cache-control", "public, max-age=315360000")
-	w.Header().Set("last-modified", lastUpdate.Format(time.RFC1123))
+	w.Header().Set("last-modified", lastUpdate.Format(http.TimeFormat))
 
 	cnt, err := io.Copy(w, imgReader)
 	if err != nil {