Code-split feature pages and admin shell with React.lazy

The dashboard (recharts, ~120 kB gzip), tracing, troubleshoot, actions and bootstrap wizard now load on demand, and the admin panel is split out of the entry chunk so anonymous visitors only download the login page (~134 kB gzip instead of ~557 kB).

src/components/common/LoadingFallback.tsx

@@ -0,0 +1,24 @@
+/*
+ * SPDX-FileCopyrightText: 2020 Stalwart Labs LLC <hello@stalw.art>
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-SEL
+ */
+
+import { Loader2 } from 'lucide-react';
+import { useTranslation } from 'react-i18next';
+
+interface LoadingFallbackProps {
+  fullScreen?: boolean;
+}
+
+export function LoadingFallback({ fullScreen }: LoadingFallbackProps) {
+  const { t } = useTranslation();
+  return (
+    <div className={`flex items-center justify-center ${fullScreen ? 'min-h-screen' : 'p-8'}`}>
+      <div className="flex flex-col items-center gap-3">
+        <Loader2 className="h-8 w-8 animate-spin text-primary" />
+        <p className="text-muted-foreground">{t('common.loading')}</p>
+      </div>
+    </div>
+  );
+}