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/src/app/_layout.tsx

@@ -15,7 +15,6 @@ import { KeyboardProvider } from "react-native-keyboard-controller";
 import { Uniwind, useResolveClassNames } from "uniwind";
 
 import { OfflineBanner } from "@/components/ui/offline-banner";
-import { ToastProvider } from "@/components/ui/toast-provider";
 import { authClient } from "@/lib/auth-client";
 import { queryPersister } from "@/lib/mmkv";
 import { applyTrackingTransparency, posthog } from "@/lib/posthog";
@@ -91,7 +90,6 @@ function AppContent() {
     <>
       <StatusBar style="light" />
       <OfflineBanner />
-      <ToastProvider />
       <Stack
         screenOptions={{
           headerShown: false,