feat: add 'Clear Selection' button

## Problem
Users can only clear their selection by clicking each selected word individually.

## Impact
Frustrating UX when users want to start over with their selection.

## Solution
1. Add `clearSelection()` method to store
2. Add "Clear" button in the UI (visible when words are selected)

```typescript
// In store
function clearSelection() {
selectedWords.value.clear()
status.value = 'selecting'
}
```

```html

Clear

```

## Priority
Medium - Quick win (~15 minutes)