feat: add responsive grid layout for mobile

## Problem
The word grid always uses 4 columns (`grid-template-columns: repeat(4, 1fr)`), making buttons too small to tap on mobile devices.

## File
`src/components/WordsGrid.vue` (lines 99-101)

## Impact
Poor mobile experience, accessibility concerns for touch targets.

## Solution
Add responsive breakpoints:
```css
@media (max-width: 600px) {
.words-grid {
grid-template-columns: repeat(2, 1fr);
}
}
@media (min-width: 601px) and (max-width: 900px) {
.words-grid {
grid-template-columns: repeat(3, 1fr);
}
}
```

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