feat: add telephonyEnabled to ui-flags endpoint

TELEPHONY_ENABLED env var (default true). When set to false, frontend
hides call center nav and shows CRM-only mode.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/config/setup-state.controller.ts

@@ -67,6 +67,7 @@ export class SetupStateController {
     uiFlags() {
         return {
             setupManaged: process.env.HELIX_SETUP_MANAGED === 'true',
+            telephonyEnabled: process.env.TELEPHONY_ENABLED !== 'false', // default true
         };
     }
 }