Prepare product layout and reusable Flutter theme

Relocate the existing Astro website and preserve its assets and hosting configuration. Add the consumer/backend scaffold and themed Flutter package with consumer tests. Related to #1.

website/astro.config.mjs

@@ -0,0 +1,6 @@
+import { defineConfig } from 'astro/config';
+export default defineConfig({
+  site: 'https://sxchnge.s-deshevykh.chatgpt.site',
+  output: 'static',
+  trailingSlash: 'always',
+});