Remove all obsolete authentication, settings, admin, and demo-related components and pages

- Eliminated redundant components, pages, and layouts related to authentication (`login`, `register`, `password-reset`, etc.), user settings, admin, and demos.
- Simplified the frontend structure by removing unused dynamic imports, forms, and test code.
- Refactored configurations and metadata imports to exclude references to removed features.
- Streamlined the project for future development and improved maintainability by discarding legacy and unused code.

frontend/src/app/[locale]/dev/components/page.tsx

@@ -0,0 +1,27 @@
+/**
+ * Component Showcase Page
+ * Development-only page to preview all shadcn/ui components
+ * Access: /dev/components
+ */
+
+import type { Metadata } from 'next';
+import dynamic from 'next/dynamic';
+
+// Code-split heavy dev component (787 lines)
+const ComponentShowcase = dynamic(
+  () => import('@/components/dev/ComponentShowcase').then((mod) => mod.ComponentShowcase),
+  {
+    loading: () => (
+      <div className="p-8 text-center text-muted-foreground">Loading components...</div>
+    ),
+  }
+);
+
+export const metadata: Metadata = {
+  title: 'Component Showcase | Dev',
+  description: 'Preview all design system components',
+};
+
+export default function ComponentShowcasePage() {
+  return <ComponentShowcase />;
+}