feat: introduce new app routes with modular structure and enhanced features

Added modular app routes including `login`, `dashboard`, `categories`, `trends`, and `upload`. Introduced reusable components such as `ContentList`, `ContentSkeleton`, and `AppSidebar` for improved UI consistency. Enhanced authentication with `AuthProvider` and implemented lazy loading, dynamic layouts, and infinite scrolling for better performance.

frontend/src/app/not-found.tsx

@@ -0,0 +1,34 @@
+"use client";
+
+import Link from "next/link";
+import { Button } from "@/components/ui/button";
+import { Home, AlertCircle } from "lucide-react";
+
+export default function NotFound() {
+  return (
+    <div className="min-h-screen flex flex-col items-center justify-center bg-zinc-50 dark:bg-zinc-950 px-4">
+      <div className="text-center space-y-6 max-w-md">
+        <div className="flex justify-center">
+          <div className="bg-red-100 dark:bg-red-900/30 p-4 rounded-full">
+            <AlertCircle className="h-12 w-12 text-red-600 dark:text-red-400" />
+          </div>
+        </div>
+        <h1 className="text-4xl font-bold tracking-tight">404 - Perdu dans le troupeau ?</h1>
+        <p className="text-muted-foreground text-lg">
+          On dirait que ce mème s'est enfui. La chèvre ne l'a pas trouvé.
+        </p>
+        <div className="flex flex-col sm:flex-row gap-3 justify-center">
+          <Button asChild size="lg" className="gap-2">
+            <Link href="/">
+              <Home className="h-4 w-4" />
+              Retourner à l'accueil
+            </Link>
+          </Button>
+        </div>
+      </div>
+      <div className="mt-12 text-8xl grayscale opacity-20 select-none">
+        🐐
+      </div>
+    </div>
+  );
+}