Fix backup download link — missing file= query parameter

[uAliAmer]

Bug

Backup files cannot be downloaded from the admin panel. Clicking a backup download button navigates the browser to the page URL instead of triggering a file download, and throws a JS error:

```
Uncaught TypeError: Cannot read properties of null (reading 'addEventListener')
```

Root Cause

The download link is generated with the filename as a bare query key:

```php
// Generates: /admin/backups/?filename.zip
url('admin/backups') . '/?' . $entry
```

But the download handler checks `$_GET['file']`:

```php
if (isset($_GET['file'])) {
$filename = $_GET['file'];
// ... serve file
}
```

Since `$_GET['file']` is never set, the handler is skipped and the page renders as HTML, causing the JS error.

Fix

```php
// Generates: /admin/backups?file=filename.zip
url('admin/backups') . '?file=' . $entry
```

Two corrections in one:

1. Removed the stray `/` before `?`
2. Changed `$entry` to `file=$entry` to match what the handler expects

Test

Verified on a self-hosted instance behind an nginx reverse proxy — backup downloads work correctly after this change.