Fix compatibility issues with Qt/PyQt6 versions

- Add null check for message.message in search filter
- Handle missing colorScheme/colorSchemeChanged methods for older Qt versions
- Add display check to prevent hanging in headless environments
- Update build documentation with system package alternative
- Update PyInstaller spec for Python 3.12
- Improve run.sh script with venv management

gotify_tray/gui/models/MessagesModel.py

@@ -65,7 +65,7 @@ class MessagesProxyModel(QtCore.QSortFilterProxyModel):
             return False
         if self.text_filter:
             title = (message.title or "").lower()
-            msg = message.message.lower()
+            msg = (message.message or "").lower()
             if self.text_filter not in title and self.text_filter not in msg:
                 return False
         return True