add a 404 page

404.php

@@ -0,0 +1,12 @@
+<?php
+// Caddy's handle_errors 404 rewrites here, so set the status ourselves.
+require_once __DIR__ . '/config.php';
+
+http_response_code(404);
+$page_title = '404 · ' . SITE_TITLE;
+require __DIR__ . '/inc/header.php';
+?>
+<p class="back-link"><a href="/">&larr; go home</a></p>
+<h1>404</h1>
+<p>nothing here.</p>
+<?php require __DIR__ . '/inc/footer.php'; ?>