Refactor: Make isServerReachable asynchronous

Refactored `isServerReachable` to be a `suspend` function that uses a Retrofit `HEAD` request instead of a synchronous `HttpURLConnection`. This eliminates the risk of an ANR when checking for server connectivity.

- Created a `HealthCheckApi` interface for the `HEAD` request.
- Added a custom `OkHttpClient` with a 5-second timeout for the health check.
- Restored the `ServerUrlMapper` and URL scheme-prepending logic to maintain existing functionality.
- Fixed a duplicate import statement in `MainApplication.kt`.

Author: google-labs-jules[bot]

app/src/main/java/org/ole/planet/myplanet/MainApplication.kt

@@ -53,7 +53,6 @@ import org.ole.planet.myplanet.utilities.NetworkUtils.isNetworkConnectedFlow
 import org.ole.planet.myplanet.utilities.NetworkUtils.startListenNetworkState
 import org.ole.planet.myplanet.utilities.NetworkUtils.stopListenNetworkState
 import org.ole.planet.myplanet.utilities.ServerUrlMapper
-import org.ole.planet.myplanet.utilities.ServerUrlMapper
 import org.ole.planet.myplanet.utilities.ThemeMode
 import org.ole.planet.myplanet.utilities.VersionUtils.getVersionName