refactor to use Gitify Notification type

[setchy]

📝 Provide a description of the new feature

Currently, a lot of the logic uses the raw GitHub response types (REST and GraphQL).

I'd like to propose we refactor to use a Gitify specific Notification type (eg: GitifyNotification) that contains only the fields required for the experience (eg: title, color, icon, type, state, timestamp, url), and avoid passing around/using the raw response types.

➕ Additional Information

No response