fix: restore tree editor visibility after Tailwind v4 upgrade and add Sentry DSN build arg

- Add missing `flex` class on TreeEditorPage editor wrapper (collapsed canvas to 0 height)
- Rewrite React Flow CSS overrides to use --xy-* custom properties (v12 compat with TW4)
- Move React Flow CSS import from component to index.css (CSS layer ordering)
- Add VITE_SENTRY_DSN build arg to Dockerfile for Railway builds
- Use env var for Sentry DSN in instrument.ts with hardcoded fallback
- Add lessons learned #53-55 to CLAUDE.md

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

frontend/src/instrument.ts

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