bug: offline page "Back to the map" links to / instead of /map

## Problem

`src/app/offline/page.tsx:37-39`:

```tsx
<Link href="/">Back to the map</Link>
```

The label promises the map but delivers the homepage — extra tap for someone on a flaky connection, which is exactly who sees this page.

## Fix

One-line change: `href="/map"`.

## Acceptance criteria

- [ ] Button lands on /map