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>

proxy.ts

@@ -1,5 +1,8 @@
 import { NextRequest, NextResponse } from "next/server";
 import { SESSION_COOKIE_NAME, verifySessionToken } from "@/lib/auth/session";
+import { createLogger } from "@/lib/logger";
+
+const log = createLogger("auth");
 
 export default async function proxy(req: NextRequest) {
   const { pathname } = req.nextUrl;
@@ -13,9 +16,11 @@ export default async function proxy(req: NextRequest) {
   }
 
   if (pathname.startsWith("/api/")) {
+    log.warn("rejected unauthenticated request", { path: pathname, method: req.method });
     return NextResponse.json({ error: "unauthorized" }, { status: 401 });
   }
 
+  log.warn("redirecting unauthenticated request to login", { path: pathname });
   const loginUrl = new URL("/login", req.url);
   loginUrl.searchParams.set("from", pathname);
   return NextResponse.redirect(loginUrl);