chore: add copyright headers to web ui components

Add a per-file copyright header so ownership is visible at the file level (consistent with the root `LICENSE`).

**Mechanical change** — prepend this exact line as the first line of each file, nothing else:

```
// Copyright (c) 2026 Sub Rosa contributors
```

Add it to the following files:

- `apps/web/src/ui/Confetti.tsx`
- `apps/web/src/ui/CountUp.tsx`
- `apps/web/src/ui/Toast.tsx`
