feat(notification): support custom notification IDs

Update `NotificationSys.showNotification` to accept an optional `id`
parameter. This allows different notifications to be displayed
simultaneously by the system instead of overwriting each other.

In `SettingsFragment` and `PlayerFragment`, assign specific unique IDs
for test, share, and update notifications.

app/src/main/java/com/michatec/radio/PlayerFragment.kt

@@ -885,7 +885,8 @@ class PlayerFragment : Fragment(),
                         requireContext(),
                         "${getString(R.string.app_name)} $latestVersion",
                         getString(R.string.snackbar_update_available),
-                        intent = updateIntent
+                        intent = updateIntent,
+                        id = 1001
                     )
                 }
             }