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/globals.css

@@ -133,8 +133,22 @@ body {
   animation: pulse-glow 2s ease-in-out infinite;
 }
 
+@keyframes pulse-glow-down {
+  0%,
+  100% {
+    box-shadow: 0 0 0 0 color-mix(in srgb, var(--pn-status-down) 55%, transparent);
+  }
+  50% {
+    box-shadow: 0 0 0 4px color-mix(in srgb, var(--pn-status-down) 0%, transparent);
+  }
+}
+.status-pulse-down {
+  animation: pulse-glow-down 2s ease-in-out infinite;
+}
+
 @media (prefers-reduced-motion: reduce) {
   .status-pulse,
+  .status-pulse-down,
   .pn-toast,
   .pn-spin-in {
     animation: none;