Update learnings.md with modernization insights

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

Author: jdpigeon

.llms/learnings.md

@@ -8,3 +8,42 @@ Format: brief heading + explanation + (optional) relevant file paths.
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 <!-- Add entries below this line -->
+
+## React Router v6 + redux connect + class components: TypeScript inference gap
+
+`withRouter<P>(Component: ComponentType<P & WithRouterProps>): ComponentType<P>`
+works at runtime but TypeScript cannot infer `P` when the argument is a
+`ConnectedComponent` (react-redux). Pragmatic solution: cast in `routes.tsx` with
+a comment; or fully convert the class component to a functional component using
+`useNavigate()` directly. See `src/renderer/utils/withRouter.tsx` and `routes.tsx`.
+
+## connected-react-router → RouteChanged action pattern
+
+Instead of listening to `@@router/LOCATION_CHANGE` in epics, dispatch
+`ExperimentActions.RouteChanged(pathname)` from a `RouteChangeTracker`
+component (uses `useLocation`). Epics then filter on this action. See
+`src/renderer/epics/experimentEpics.ts` and
+`src/renderer/components/RouteChangeTracker.tsx`.
+
+## Tailwind v4 setup
+
+Tailwind v4 uses `@tailwindcss/vite` plugin (no `tailwind.config.js`, no PostCSS
+config). Add the plugin to the `renderer` section of `vite.config.ts` and
+`@import 'tailwindcss'` at the top of the main CSS file.
+
+## Shadcn/ui components live in src/renderer/components/ui/
+
+Button, Card, Dialog — plus the `cn()` utility at `src/renderer/utils/cn.ts`.
+Use these instead of Semantic UI equivalents.
+
+## Pyodide v0.27.0 URL format change
+
+v0.21.0 archive: `pyodide-build-{version}.tar.bz2`
+v0.27.0+ archive: `pyodide-{version}.tar.bz2` (no "build-" prefix).
+`InstallPyodide.js` must be updated accordingly when changing versions.
+
+## sns.tsplot removed in Seaborn 0.10
+
+`plot_conditions()` in `utils.py` used `sns.tsplot` which was removed in
+Seaborn 0.10. Replaced with `plt.plot` + `fill_between` for mean ± SEM.
+Seaborn is not imported at all now — matplotlib only for this function.