gui: serve static files with gzip/deflate compression

Before this change, the GUI server sent all static files uncompressed,
meaning the browser had to download the full size of every JS, CSS,
and HTML asset.

After this change, the GUI server uses chi's Compress middleware at
level 5, which negotiates gzip or deflate encoding based on the
client's Accept-Encoding header.

This reduces transfer sizes significantly for the web UI assets, for
example assets/index-CvfdU_RR.js is 874 KB uncompressed, and
265 KB compressed.

This is consistent with how rclone serve http, webdav, and restic
already compress their responses.

cmd/gui/gui.go

@@ -14,6 +14,7 @@ import (
 	"strings"
 	"sync"
 
+	"github.com/go-chi/chi/v5/middleware"
 	"github.com/rclone/rclone/cmd"
 	"github.com/rclone/rclone/fs"
 	"github.com/rclone/rclone/fs/rc"
@@ -191,6 +192,7 @@ For more help see [the GUI docs](/gui/).
 		if err != nil || spaHandler == nil {
 			return fmt.Errorf("failed to start GUI handler: %w", err)
 		}
+		guiServer.Router().Use(middleware.Compress(5))
 		guiServer.Router().Get("/*", spaHandler.ServeHTTP)
 		guiServer.Router().Head("/*", spaHandler.ServeHTTP)
 		guiServer.Serve()