Replace catch-all proxy with dedicated routes, add 404 page

- Dedicated proxy routes for sessions, messages, search, memory, config
- Removes [...path] catch-all that conflicted with specific routes
- Session detail + delete proxy
- Custom 404 page matching design system

app/not-found.tsx

@@ -0,0 +1,17 @@
+import Link from "next/link";
+
+export default function NotFound() {
+  return (
+    <div className="flex flex-col items-center justify-center py-20 space-y-4">
+      <div className="text-6xl font-bold" style={{ color: "var(--text-3)" }}>404</div>
+      <p className="text-sm" style={{ color: "var(--text-2)" }}>Page not found</p>
+      <Link
+        href="/"
+        className="px-4 py-2 rounded-xl text-sm font-medium"
+        style={{ background: "var(--accent-dim)", color: "var(--accent)" }}
+      >
+        Back to Pulse
+      </Link>
+    </div>
+  );
+}