Replace `sqflite` with file-based JSON storage

[Sembauke]

We currently use the sqflite package to persist article data in the app. This is more complex than our needs require. The goal is to replace sqflite with a simpler file-based JSON storage system.

Requirements

• Store all articles as a list in a single articles.json file inside the app’s documents directory.
• Ensure the storage layer gracefully handles the case where articles.json does not yet exist.
• Provide helper functions to:
• Load all articles from the JSON file at app startup.
• Save the current in-memory list of articles back to the JSON file.
• Add, update, and delete articles in memory, then persist changes to disk.

Acceptance criteria

• App can be installed fresh and run without sqflite.
• A clean install correctly creates articles.json when the first article is saved.
• After migration, existing user data is preserved and stored in the new JSON file format.
• All bookmark tests pass with the new JSON storage implementation.