Add a minimal 404 page matching the app's style

Two boundaries share the same content: src/app/not-found.tsx catches
genuinely unmatched URLs (rendered bare in the root layout), and
(app)/not-found.tsx catches notFound() calls from within app routes
(already used by scripts/[scriptId] and runs/[runId]) so it renders
nested inside AppLayout, keeping nav and footer visible.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/app/(app)/not-found.tsx

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+import { NotFoundContent } from "@/components/layout/not-found-content";
+
+export default function NotFound() {
+  return <NotFoundContent />;
+}

src/app/not-found.tsx

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+import { NotFoundContent } from "@/components/layout/not-found-content";
+
+export default function NotFound() {
+  return <NotFoundContent />;
+}

src/components/layout/not-found-content.tsx

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+import Link from "next/link";
+import { Home } from "lucide-react";
+import { buttonVariants } from "@/components/ui/button";
+import { cn } from "@/lib/utils";
+
+export function NotFoundContent() {
+  return (
+    <div className="mx-auto flex max-w-md flex-col items-center gap-3 py-24 text-center">
+      <span className="text-primary font-mono text-6xl font-semibold tracking-tight">
+        404
+      </span>
+      <h1 className="font-heading text-xl font-medium">Page not found</h1>
+      <p className="text-muted-foreground text-sm">
+        The page you&rsquo;re looking for doesn&rsquo;t exist or may have been
+        moved.
+      </p>
+      <Link
+        href="/"
+        className={cn(
+          buttonVariants({ variant: "outline", size: "sm" }),
+          "mt-2",
+        )}
+      >
+        <Home className="size-3.5" />
+        Back to dashboard
+      </Link>
+    </div>
+  );
+}