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/Dockerfile

@@ -12,9 +12,11 @@ RUN npm ci
 # Copy source code
 COPY . .
 
-# Build argument for API URL (set at build time)
+# Build arguments (set at build time)
 ARG VITE_API_URL
+ARG VITE_SENTRY_DSN
 ENV VITE_API_URL=$VITE_API_URL
+ENV VITE_SENTRY_DSN=$VITE_SENTRY_DSN
 
 # Build the application
 RUN npm run build