Use textual's `.open_url` method here

so textual have this `.open_url` method: https://textual.textualize.io/api/app/#textual.app.App.open_url
replace with this, also remove unused imports.
https://github.com/stabldev/torrra/blob/e4cd60570506adb6a5fb76e44091109dc61b42af/src/torrra/widgets/search.py#L106

replace this with:
```py
self.app.open_url(resolved_magnet_uri)
```