Add message filtering by priority and subject, with CRITICAL always visible

- Implement MessagesProxyModel for client-side filtering
- Add priority filter buttons (LOW 0-3, NORMAL 4-8, HIGH 9, CRITICAL 10 always shown)
- Add subject filter menu with checkable actions for message titles
- Add Remove Filters button to reset all filters
- Restore priority 10 persistent notification setting in options
- Fix settings dialog errors and update UI layouts
- Ensure CRITICAL priority messages cannot be filtered out but can toggle persistent pop-ups

gotify_tray/gui/models/__init__.py

@@ -5,4 +5,9 @@ from .ApplicationModel import (
     ApplicationProxyModel,
     ApplicationItemDataRole,
 )
-from .MessagesModel import MessagesModelItem, MessagesModel, MessageItemDataRole
+from .MessagesModel import (
+    MessagesModelItem,
+    MessagesModel,
+    MessagesProxyModel,
+    MessageItemDataRole,
+)