Add Standalone Login Page Renderer

Add a hash-bound, no-store login document with focused C and Node host
tests. Keep rendering inert until the 8D.3 authentication cutover.

src/web_login_ui.h

@@ -0,0 +1,10 @@
+/* SPDX-License-Identifier: GPL-3.0-only */
+#pragma once
+
+#include "esp_err.h"
+#include "esp_http_server.h"
+
+/* Standalone public login document for the future atomic 8D.3 cutover.
+ * Rendering only: no authentication, URI registration, or session allocation.
+ * Do not expose this page until its protected API and application routes exist. */
+esp_err_t web_login_ui_send_response(httpd_req_t *request);