Add an opt-in SSR mode with a Node adapter behind PUBLIC_SSR

svelte.config.js

@@ -1,11 +1,21 @@
-import adapter from "@sveltejs/adapter-static";
+import { loadEnv } from "vite";
+import adapterNode from "@sveltejs/adapter-node";
+import adapterStatic from "@sveltejs/adapter-static";
+
+// PUBLIC_SSR picks the deployment target: a Node server that renders pages
+// (yes) or a static single-page bundle (no, the default). The same flag turns
+// SSR on in +layout.ts.
+const env = loadEnv(
+  process.env.NODE_ENV ?? "production",
+  process.cwd(),
+  "PUBLIC_",
+);
+const ssr = (process.env.PUBLIC_SSR ?? env.PUBLIC_SSR) === "yes";
 
 /** @type {import('@sveltejs/kit').Config} */
 const config = {
   kit: {
-    adapter: adapter({
-      fallback: "index.html",
-    }),
+    adapter: ssr ? adapterNode() : adapterStatic({ fallback: "index.html" }),
   },
 };