Add server-generated /config.js; add error boundary

web/public/config.js

@@ -0,0 +1,9 @@
+// Configuration injected by the ntfy server.
+//
+// This file is just an example. It is removed during the build process.
+// The actual config is dynamically generated server-side.
+
+var config = {
+    appRoot: "/",
+    disallowedTopics: ["docs", "static", "file", "app", "settings"]
+};