refactor(native): replace custom toast implementation with burnt

Swap the hand-rolled `ToastView`/`ToastProvider`/queue system for the
`burnt` library, which delegates to native iOS/Android toast APIs.

- Add `burnt@0.13.0` dependency
- Delete `toast.tsx` and `toast-provider.tsx`; remove `<ToastProvider />`
  from `_layout.tsx`
- Rewrite `lib/toast.ts` to wrap `burnt` — maps success/error/info/warning
  to burnt presets and haptic types; duration converted from ms to seconds

apps/native/package.json

@@ -34,6 +34,7 @@
     "@tanstack/react-query": "catalog:",
     "@tanstack/react-query-persist-client": "5.90.24",
     "better-auth": "catalog:",
+    "burnt": "0.13.0",
     "date-fns": "catalog:",
     "expo": "55.0.6",
     "expo-application": "55.0.9",