Bake PUBLIC_SSR into the build instead of reading it at runtime

vite.config.ts

@@ -1,5 +1,15 @@
 import tailwindcss from "@tailwindcss/vite";
 import { sveltekit } from "@sveltejs/kit/vite";
-import { defineConfig } from "vite";
+import { defineConfig, loadEnv } from "vite";
 
-export default defineConfig({ plugins: [tailwindcss(), sveltekit()] });
+// PUBLIC_SSR is a build-time decision (it also picks the adapter in
+// svelte.config.js), so it is compiled in as a constant rather than read from
+// the environment when the server starts.
+export default defineConfig(({ mode }) => {
+  const env = loadEnv(mode, process.cwd(), "PUBLIC_");
+  const ssr = (process.env.PUBLIC_SSR ?? env.PUBLIC_SSR) === "yes";
+  return {
+    plugins: [tailwindcss(), sveltekit()],
+    define: { __SQUALK_SSR__: JSON.stringify(ssr) },
+  };
+});