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/api/auth/logout/route.ts

@@ -1,8 +1,9 @@
 import { NextResponse } from "next/server";
 import { SESSION_COOKIE_NAME } from "@/lib/auth/session";
+import { withRouteLogging } from "@/lib/api/with-route-logging";
 
-export async function POST() {
+export const POST = withRouteLogging("auth.logout", async () => {
   const res = NextResponse.json({ ok: true });
   res.cookies.delete(SESSION_COOKIE_NAME);
   return res;
-}
+});