[Toast] Animation duration coupled to CSS via parsed styles

## Component
Toast

## Problem
File: Toast.tsx lines 28-29

JS is coupled to CSS values:
```tsx
const ANIMATION_OPEN_TIME_MS = Number.parseInt(styles.enterDuration);
const ANIMATION_CLOSE_TIME_MS = Number.parseInt(styles.exitDuration);
```

## Suggested Fix
Use CSS variables or define constants in one place.

## Impact
Fragile coupling between JS and CSS