fix: hide chat notification content on lock screen (#333)

Set `NotificationCompat.VISIBILITY_PRIVATE` on the chat reply
notification builder. This prevents the first 100 characters of a
message from leaking to the lock screen, addressing security concern
SEC-09.

Resolves #262

app/src/main/java/com/m57/hermescontrol/notification/ChatNotificationService.kt

@@ -113,6 +113,7 @@ class ChatNotificationService : Service() {
                 .setContentText(text)
                 .setStyle(NotificationCompat.BigTextStyle().bigText(text))
                 .setPriority(NotificationCompat.PRIORITY_HIGH)
+                .setVisibility(NotificationCompat.VISIBILITY_PRIVATE)
                 .setAutoCancel(true)
                 .setContentIntent(buildContentIntent(sessionId))