freeze build timestamp via an environment variable

lib/config/constants.ts

@@ -1,2 +1,5 @@
 // Next.js constants (not needed in frontend)
 export const NOTES_DIR = "./notes";
+
+// normalize the timestamp saved when building/deploying (see next.config.js) and fall back to right now:
+export const RELEASE_DATE = new Date(process.env.RELEASE_DATE ?? Date.now()).toISOString();