refactor: move IS_PLATFORM to config file for both frontend and backend

The reason we couldn't place it in shared/modelConstants.js is that the
frontend uses Vite which requires import.meta.env for environment variables,
while the backend uses process.env. Therefore, we created separate config files
for the frontend (src/constants/config.ts) and backend (server/constants/config.js).

shared/modelConstants.js

@@ -62,11 +62,4 @@ export const CODEX_MODELS = {
   ],
 
   DEFAULT: 'gpt-5.2'
-};
-
-
-/**
- * Environment Flag: Is Platform
- * Indicates if the app is running in Platform mode (hosted) or OSS mode (self-hosted)
- */
-export const IS_PLATFORM = import.meta.env.VITE_IS_PLATFORM === 'true';
+};