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>

CLAUDE.md

@@ -337,6 +337,12 @@ navigate(`/trees/${newTree.id}/edit`)
 
 **52. Mobile scroll-to-top — use `scrollIntoView`, not `window.scrollTo`:** Mobile browsers (iOS Safari, Firefox Android) often ignore `window.scrollTo()`. Use a ref at the top of the page and call `ref.current.scrollIntoView({ behavior: 'smooth', block: 'start' })` instead. Trigger via `useEffect` on the state change (not inline with `setState`) so the DOM has committed before scrolling.
 
+**53. Flex height chain — every ancestor must be a flex container for `flex-1` to work:** If a child uses `flex-1` to fill its parent, the parent MUST have `display: flex` (the `flex` class). A missing `flex` on any wrapper div breaks the entire height chain, causing React Flow (and any `h-full` descendant) to collapse to 0 height. Debug with: `let el = document.querySelector('.react-flow'); while(el) { console.log(el.getBoundingClientRect().height, el.className); el = el.parentElement; }`. The break is where height drops to 0. Common symptom: React Flow error `"parent container needs a width and a height"`.
+
+**54. React Flow CSS in Tailwind v4 — import in `index.css`, not component JS:** With `@tailwindcss/vite`, importing `@xyflow/react/dist/style.css` inside a component file causes the plugin to process/wrap it in a cascade layer, lowering specificity. Import it in `index.css` instead: `@import '@xyflow/react/dist/style.css';` after `@import 'tailwindcss';`. Override dark theme using `--xy-*` CSS custom properties (e.g., `--xy-edge-stroke-default`) on `.react-flow.dark`, NOT old-style direct property selectors like `.react-flow__edge-path { stroke: ... }`.
+
+**55. App shell height chain for full-height pages (tree editor, procedural editor):** The CSS Grid app shell (`app-shell`) → `.main-content` → page component chain must preserve height. `.main-content` is a grid cell with implicit height from `1fr`. Pages using React Flow or other full-height layouts need every wrapper div between `.main-content` and the canvas to either use `flex` + `flex-1` + `min-h-0` or explicit `h-full`. Adding ANY wrapper div (e.g., for animations, transitions) without proper height classes will collapse the canvas to 0.
+
 ---
 
 ## RBAC & Permissions