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/components/layout/AppLayout.tsx

@@ -183,7 +183,7 @@ export function AppLayout() {
       )}
 
       {/* Main Content */}
-      <main className="main-content overflow-y-auto">
+      <main className="main-content">
         <EmailVerificationBanner />
         <Outlet />
       </main>