feat(layout): login, uygulama kabuğu, alt menü, hızlı ekle ve dashboard

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/app/not-found.tsx

@@ -0,0 +1,16 @@
+import Link from "next/link";
+import { Button } from "@/components/ui/button";
+import { tr } from "@/i18n/tr";
+
+export default function NotFound() {
+  return (
+    <main className="flex flex-1 flex-col items-center justify-center gap-4 p-8 text-center">
+      <p className="text-6xl font-semibold text-primary tabular-nums">404</p>
+      <h1 className="text-2xl font-semibold tracking-tight">{tr.errors.notFoundTitle}</h1>
+      <p className="max-w-md text-muted-foreground">{tr.errors.notFoundDescription}</p>
+      <Button asChild>
+        <Link href="/">{tr.errors.backHome}</Link>
+      </Button>
+    </main>
+  );
+}