refactor: Centralize platform mode detection using IS_PLATFORM constant; use token from Auth context in WebSocket connection

shared/modelConstants.js

@@ -63,3 +63,10 @@ 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';