Add 403 + 500 error templates (todo §0); render 500 via app error handler

src/app.ts

@@ -1,4 +1,4 @@
-import { createServer, type Server } from "node:http";
+import { createServer, type Server, type ServerResponse } from "node:http";
 import { dirname, join } from "node:path";
 import { fileURLToPath } from "node:url";
 import * as ejs from "ejs";
@@ -18,6 +18,11 @@ export function createApp(options: AppOptions = {}): Server {
   const render = (view: string, data: Record<string, unknown>): Promise<string> =>
     ejs.renderFile(join(viewsDir, `${view}.ejs`), data);
 
+  const sendHtml = (res: ServerResponse, status: number, html: string): void => {
+    res.writeHead(status, { "content-type": "text/html; charset=utf-8" });
+    res.end(html);
+  };
+
   return createServer(async (req, res) => {
     try {
       if (req.method !== "GET" && req.method !== "HEAD") {
@@ -33,17 +38,22 @@ export function createApp(options: AppOptions = {}): Server {
       }
 
       if (pathname === "/") {
-        res.writeHead(200, { "content-type": "text/html; charset=utf-8" });
-        res.end(await render("index", { title: "Plainpages" }));
+        sendHtml(res, 200, await render("index", { title: "Plainpages" }));
         return;
       }
 
-      res.writeHead(404, { "content-type": "text/html; charset=utf-8" });
-      res.end(await render("404", { title: "Not found" }));
+      sendHtml(res, 404, await render("404", { title: "Not found" }));
     } catch (err) {
       console.error(err);
-      if (!res.headersSent) res.writeHead(500, { "content-type": "text/plain; charset=utf-8" });
-      res.end("Internal Server Error");
+      if (res.headersSent) return void res.end(); // a partial body is already on the wire
+      try {
+        // Render first: if the error page itself fails, headers stay unsent and we
+        // fall back to plain text below rather than emit a half-written response.
+        sendHtml(res, 500, await render("500", { title: "Server error" }));
+      } catch (renderErr) {
+        console.error(renderErr);
+        res.writeHead(500, { "content-type": "text/plain; charset=utf-8" }).end("Internal Server Error");
+      }
     }
   });
 }