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

AGENTS.md

@@ -0,0 +1,19 @@
+# Agent Guidelines
+
+## Type Checking
+Run `pyright .` in the project root to perform static type checking with Pyright. Address any critical errors before committing changes.
+
+Note: Pyright may report many Qt-related type issues that are false positives due to PyQt6 stubs limitations. Focus on logical errors rather than Qt API mismatches.
+
+To ignore PyQt6-related errors, create a `pyrightconfig.json` in the project root with:
+```json
+{
+  "reportOptionalMemberAccess": false,
+  "reportAttributeAccessIssue": false,
+  "reportIncompatibleMethodOverride": false,
+  "reportArgumentType": false,
+  "reportAssignmentType": false,
+  "reportReturnType": false
+}
+```
+This suppresses common PyQt6 false positives while still catching real issues.