Modernize demo apps

[mbrandonw]

We have a lot of big things coming to this library soon, so I wanted to take some time to modernize some of the demos a bit. Some of the things we have done:

• Upgraded IDs of tables to be UUIDs instead of auto incrementing integers. The former plays better with synchronizing across devices where it is quite important to have globally unique IDs.
• Moved logic of complex features (reminders lists, reminders detail and search) to their own @Observable model. This helped a great deal with animation glitches, testing and the views were becoming a nightmare to work with.
• Wrote some tests for the gnarly logic in a few features.
• Don't run any code in the entry point of the app when running in a preview context. Xcode is not capable of making a database connection to a file on disk during previews.
• Test databases are now persisted to disk in the temporary directory instead of run in-memory. This creates a more realistic environment for tests to run in a production environment.
• Changed database seeding so that it happens automatically in previews and provided an option in-app for seeding from the simulator. Tests also have their own seeding function so that seeds in previews/simulators are not conflated with previews.
• Controlled the database date functions so that we can better test queries.