refactor: split playback proxy logic into separate handler files

internal/playback/handler/proxy_request.go

@@ -0,0 +1,21 @@
+package handler
+
+import (
+	"context"
+	netutil "mal/pkg/net"
+	"net/http"
+)
+
+func newProxyRequest(ctx context.Context, targetURL string, referer string) (*http.Request, error) {
+	req, err := http.NewRequestWithContext(ctx, http.MethodGet, targetURL, nil)
+	if err != nil {
+		return nil, err
+	}
+
+	if referer != "" {
+		req.Header.Set("Referer", referer)
+	}
+	req.Header.Set("User-Agent", netutil.Firefox121)
+
+	return req, nil
+}