Feat: Frontend hardening

Frontend/src/entry-client.tsx

@@ -1,4 +1,21 @@
+// Path: Frontend/src/entry-client.tsx
+
 // @refresh reload
+import type { JSX } from "solid-js";
 import { mount, StartClient } from "@solidjs/start/client";
 
-mount(() => <StartClient />, document.getElementById("app")!);
+const getAppRoot = (): HTMLElement => {
+	const appRoot = document.getElementById("app");
+
+	if (!(appRoot instanceof HTMLElement)) {
+		throw new Error("App root element '#app' was not found.");
+	}
+
+	return appRoot;
+};
+
+const mountApp = (): void => {
+	mount((): JSX.Element => <StartClient />, getAppRoot());
+};
+
+mountApp();