Add js web stuff to landing page + documentation

go/jsruntime/env.ts

@@ -0,0 +1,16 @@
+// Compile-time deployment environment: "development" | "staging" | "production".
+//
+// `__ENV_TYPE__` is substituted at bundle time by esbuild's `define`, fed the Go
+// compile-time constant `internal/constants.AppEnvironment` (see the
+// `esbuildDefine` helper in internal/bundler). The production bundles and the dev
+// HMR per-module transform both define it. The build-time SSR render of the
+// public pages does NOT, so the `typeof` guard yields "" there — the env badge
+// renders only after the client takeover, matching the prior runtime behavior.
+declare const __ENV_TYPE__: string;
+
+export const ENV_TYPE: string = typeof __ENV_TYPE__ !== "undefined" ? __ENV_TYPE__ : "";
+
+// True in every environment except production (and the badge-less SSR render).
+export function isNonProdEnv(): boolean {
+    return ENV_TYPE !== "" && ENV_TYPE !== "production";
+}