Add consistent API request logging and a themed 404 page, bump to 0.4.0

Every app/api route handler now goes through withRouteLogging, which logs
a correlated reqId/method/path/status/duration for each request and turns
any uncaught error into a logged stack trace plus a clean JSON 500 instead
of Next's default opaque failure. proxy.ts logs rejected auth attempts, and
instrumentation.ts's onRequestError catches anything that still escapes a
route handler. Also adds a minimal not-found page matching the app's card
styling.

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

app/not-found.tsx

@@ -0,0 +1,26 @@
+import Link from "next/link";
+import type { Metadata } from "next";
+import { Button } from "@/components/ui/button";
+import { Card, CardContent } from "@/components/ui/card";
+import { BrandMark } from "@/components/layout/BrandMark";
+
+export const metadata: Metadata = { title: "Not found" };
+
+export default function NotFound() {
+  return (
+    <div className="flex min-h-dvh items-center justify-center p-4">
+      <Card className="bp-glass w-full max-w-sm">
+        <CardContent className="flex flex-col items-center gap-4 py-4 text-center">
+          <BrandMark size={32} />
+          <div className="space-y-1">
+            <p className="bp-readout bp-gradient-text text-4xl font-semibold">404</p>
+            <p className="text-sm text-muted-foreground">This page doesn&apos;t exist.</p>
+          </div>
+          <Button asChild size="sm">
+            <Link href="/">Back to dashboard</Link>
+          </Button>
+        </CardContent>
+      </Card>
+    </div>
+  );
+}