feat: add themed 404 page

Flatlined pulse trace mirrors the PulseNode brand mark - a route
that isn't monitored here reads as "no pulse" rather than a generic
error page.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WpxhtQY3CExQdMs4j7MmJe

app/not-found.tsx

@@ -0,0 +1,55 @@
+import Link from "next/link";
+import type { Metadata } from "next";
+
+export const metadata: Metadata = {
+  title: "Not Found",
+  description: "No pulse on this route.",
+};
+
+export default function NotFound() {
+  return (
+    <main className="flex min-h-screen flex-col items-center justify-center gap-8 px-6 text-center">
+      <svg
+        width="320"
+        height="64"
+        viewBox="0 0 320 64"
+        fill="none"
+        aria-hidden="true"
+        className="w-full max-w-[320px]"
+      >
+        <polyline
+          points="0,32 30,32 40,10 50,54 60,24 72,32 140,32"
+          fill="none"
+          stroke="var(--pn-accent)"
+          strokeWidth="2"
+          strokeLinecap="round"
+          strokeLinejoin="round"
+        />
+        <line
+          x1="140"
+          y1="32"
+          x2="300"
+          y2="32"
+          stroke="var(--pn-status-down)"
+          strokeWidth="2"
+          strokeLinecap="round"
+          opacity="0.55"
+        />
+        <circle cx="300" cy="32" r="4" fill="var(--pn-status-down)" className="status-pulse-down" />
+      </svg>
+
+      <div className="flex flex-col gap-3">
+        <p className="font-display text-7xl font-semibold tracking-tight text-fg">404</p>
+        <p className="text-base font-medium text-fg">No pulse on this route.</p>
+        <p className="text-sm text-fg-muted">Nothing here is being monitored.</p>
+      </div>
+
+      <Link
+        href="/"
+        className="rounded-md border border-border px-4 py-2 text-sm font-medium text-fg transition-colors hover:border-accent hover:text-accent"
+      >
+        Back to dashboard
+      </Link>
+    </main>
+  );
+}