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/src/components/AccountDialog.astro

@@ -0,0 +1,21 @@
+---
+import { copy, type Lang } from '../i18n/copy';
+const { lang } = Astro.props as { lang: Lang };
+const t = copy[lang];
+---
+
+<dialog
+  id="account-dialog"
+  aria-labelledby="account-title"
+  aria-describedby="account-description"
+>
+  <button class="dialog-x" data-close aria-label={t.dialogClose}>×</button><span
+    class="dialog-symbol"
+    aria-hidden="true">↗</span
+  ><p class="eyebrow">SXCHNGE / {t.demo}</p><h2 id="account-title">
+    {t.authTitle}
+  </h2><p id="account-description">{t.authBody}</p><button
+    class="button button-dark"
+    data-close
+    autofocus>{t.close}<span aria-hidden="true">↗</span></button>
+</dialog>