docs(ai): improve onboarding steps and fix lint

- Fix `npm run lint` failure by replacing inline style with CSS class in `js/ui/components/SearchFilterModal.js`.
- Fix `npm run lint:tokens` failure by defining `--size-40rem` in `css/tokens.css` and using it in `css/tailwind.source.css`.
- Update `CONTRIBUTING.md` with troubleshooting tips for unit test hangs and linting failures.
- Attempt to update `caniuse-lite` to silence build warning (may persist due to environment/cache).

css/tokens.css

@@ -321,6 +321,7 @@
   --size-20rem: 20rem;
   --size-28rem: 28rem;
   --size-34rem: 34rem;
+  --size-40rem: 40rem;
   --size-45rem: 45rem;
   --size-70rem: 70rem;