Use JSON for errors and page response

json.go

@@ -0,0 +1,14 @@
+package main
+
+import (
+	"encoding/json"
+	"net/http"
+)
+
+func sendJSON(w http.ResponseWriter, v any) {
+	w.Header().Set("Content-Type", "application/json")
+
+	enc := json.NewEncoder(w)
+	enc.SetEscapeHTML(false)
+	_ = enc.Encode(v)
+}