Add persistent notifications for priority 10 messages

- Implement custom PersistentNotification widget with flashing background
- Add settings for persistent priority 10 notifications and sound control
- Modify notification logic to show persistent pop-ups for priority 10
- Allow closing all persistent notifications via tray icon click
- Add AGENTS.md with type checking guidelines
- Configure pyright to suppress PyQt6 false positives
- Update UI in settings dialog for new options
- Add notification sound file

gotify_tray/database/default_settings.py

@@ -16,6 +16,8 @@ DEFAULT_SETTINGS = {
     "tray/notifications/duration_ms": 5000,
     "tray/notifications/icon/show": True,
     "tray/notifications/click": True,
+    "tray/notifications/priority10_persistent": True,
+    "tray/notifications/sound_only_priority10": True,
     "tray/icon/unread": False,
     "watchdog/enabled": True,
     "watchdog/interval/s": 60,