[Enhancement] Plugin System

I would love to write some plugins for this app.

Here's what I would make first:
Homescreen widget to show a random note (or line) of the day
Parsing capabilities for more than just the simplenote json file

Maybe in the future: AI-tagging

No files, tests, or entry points are identified. Start by surveying the Android app architecture and existing import or extension-related code, then clarify the plugin boundary and which of the proposed capabilities is in scope. Done requires an agreed plugin design and a defined first capability.

Written by the indexing model from the issue text.