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/pages/TreeEditorPage.tsx

@@ -517,7 +517,7 @@ export function TreeEditorPage() {
   }
 
   return (
-    <div className="flex h-full overflow-hidden">
+    <div className="flex h-[calc(100vh-56px)] overflow-hidden">
       {/* Main content column */}
       <div className="flex min-w-0 flex-1 flex-col overflow-hidden">
 
@@ -805,7 +805,7 @@ export function TreeEditorPage() {
       )}
 
       {/* Main Editor */}
-      <div className="min-h-0 flex-1 overflow-hidden">
+      <div className="flex min-h-0 flex-1 overflow-hidden">
         <TreeEditorLayout
           isMobile={isMobile}
           isMetadataOpen={isMetadataOpen}