consolidate environment variable parsing

lib/config/constants.ts

@@ -5,3 +5,8 @@ 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();
+
+// detect if running locally via `next dev` (phase is checked in next.config.js)
+export const IS_DEV_SERVER = process.env.IS_DEV_SERVER === "true";
+
+export const BUILD_ENV = process.env.VERCEL_ENV || process.env.NEXT_PUBLIC_VERCEL_ENV || process.env.NODE_ENV;