fix(#214): notification tap now navigates to correct session

Root cause: MainActivity had no onNewIntent override. When the app was
backgrounded and a notification was tapped, Android brought the existing
task to the foreground and delivered the intent via onNewIntent — but
since it wasn't overridden, the session ID from the notification was
silently dropped and the user saw the last-active session.

Fix:
1. Changed notificationSessionId from a local val to a Compose
   mutableStateOf so it's observable across recompositions.
2. Added onNewIntent override that reads the session ID from the
   incoming intent and updates the observable state.
3. Added FLAG_ACTIVITY_NEW_TASK | FLAG_ACTIVITY_CLEAR_TOP to the
   notification PendingIntent for reliable task management.

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

@@ -153,9 +153,11 @@ class ChatNotificationService : Service() {
     }
 
     private fun buildContentIntent(sessionId: String?): PendingIntent {
-        val intent = Intent(this, MainActivity::class.java)
-        if (!sessionId.isNullOrBlank()) {
-            intent.putExtra(NotificationReplyReceiver.EXTRA_SESSION_ID, sessionId)
+        val intent = Intent(this, MainActivity::class.java).apply {
+            flags = Intent.FLAG_ACTIVITY_NEW_TASK or Intent.FLAG_ACTIVITY_CLEAR_TOP
+            if (!sessionId.isNullOrBlank()) {
+                putExtra(NotificationReplyReceiver.EXTRA_SESSION_ID, sessionId)
+            }
         }
         return PendingIntent.getActivity(
             this,