Add 404 page

.vscode/settings.json

@@ -0,0 +1,3 @@
+{
+  "editor.formatOnSave": false
+}

src/pages/404.astro

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+---
+import Layout from "../layouts/BaseLayout.astro";
+---
+
+<Layout>
+	<div style="text-align:center;">
+		<h1>404</h1>
+		<p><strong>Page not found</strong></p>
+		<p>
+			<small>
+				If you see this message, please
+				<a href=`mailto:valentin@popov.link?subject=${encodeURIComponent('I found a broken page')}`>
+					let me know
+				</a>
+			</small>
+		</p>
+	</div>
+</Layout>