refactor(proxy): remove usage of fmt

There is no need to use the heavy machinery (fmt) when we can simply use
string concatenation instead.

internal/ui/proxy.go

@@ -8,11 +8,11 @@ import (
 	"crypto/sha256"
 	"encoding/base64"
 	"errors"
-	"fmt"
 	"log/slog"
 	"net/http"
 	"net/url"
 	"path"
+	"strconv"
 	"time"
 
 	"miniflux.app/v2/internal/config"
@@ -128,7 +128,7 @@ func (h *handler) mediaProxy(w http.ResponseWriter, r *http.Request) {
 		)
 
 		// Forward the status code from the origin.
-		http.Error(w, fmt.Sprintf("Origin status code is %d", resp.StatusCode), resp.StatusCode)
+		http.Error(w, "Origin status code is "+strconv.Itoa(resp.StatusCode), resp.StatusCode)
 		return
 	}