Gzip the text responses

Nothing was compressed: the list page shipped ~123 KB of HTML, CSS and JS in
the clear on every cold load, which is the wrong bill for a phone on mobile
data. A stdlib middleware compresses the four content types this server
actually serves in bulk and leaves woff2 alone, since it is already compressed.

Measured on the live server: style.css 21.8 KB -> 6.5, htmx 50.9 -> 16.4,
filter.js 7.6 -> 2.9.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

backend/main.go

@@ -179,7 +179,7 @@ func newRouter(store *Store, cfg Config) http.Handler {
 		web.register(mux)
 	}
 
-	return withCORS(cfg.AllowedOrigins, guardEmptyUserscriptToken(mux))
+	return withCORS(cfg.AllowedOrigins, withGzip(guardEmptyUserscriptToken(mux)))
 }
 
 // guardEmptyUserscriptToken heads off ServeMux's own path-cleaning redirect: