Fix back gesture from connection details skipping list

On phones, the Connections list and Connection Details share a single
ModalBottomSheet. System back was consumed by the sheet's default dismiss
handler, closing the sheet and returning to Dashboard. The in-app back
button just cleared selectedConnectionId, returning to the list.

Add a BackHandler that runs only while a connection is selected, matching
the in-app back button behavior. Swipe-to-dismiss remains unchanged.

app/src/main/java/io/nekohasekai/sfa/compose/MainActivity.kt

@@ -8,6 +8,7 @@ import android.net.Uri
 import android.net.VpnService
 import android.os.Build
 import android.os.Bundle
+import androidx.activity.compose.BackHandler
 import androidx.activity.compose.setContent
 import androidx.activity.enableEdgeToEdge
 import androidx.activity.result.contract.ActivityResultContracts
@@ -1107,6 +1108,10 @@ class MainActivity :
                 }
             }
 
+            BackHandler(enabled = selectedConnectionId != null) {
+                selectedConnectionId = null
+            }
+
             ModalBottomSheet(
                 onDismissRequest = {
                     showConnectionsSheet = false