fix: remove hardcoded Sentry DSN fallback — use env var only

VITE_SENTRY_DSN is already set in Railway as a build arg. The hardcoded
fallback was unnecessary and triggered GitHub secret scanning alerts.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/instrument.ts

@@ -1,7 +1,7 @@
 import * as Sentry from "@sentry/react";
 
 Sentry.init({
-  dsn: import.meta.env.VITE_SENTRY_DSN || "https://23937b8c0cea2484f6a9d5b97d0b7d4b@o4511005918887936.ingest.us.sentry.io/4511005926883328",
+  dsn: import.meta.env.VITE_SENTRY_DSN,
   environment: import.meta.env.MODE,
 
   integrations: [