Domain: UseCases, ContentItem, Id and Instant Logic, expand tests

[lyzlovazlata]

1. Add deafult usecases with notes
2. Edit ContentItem - add id of each
3. Add ContentItemUseCases
4. Sorted UseCases packages
5. Add and expand tests

[allnes]

Thanks for the work here. There is a lot of useful progress in this PR.

For review, I think it would be much easier if this were split into a few smaller PRs. Right now this branch seems to include several different kinds of changes at once:

• domain model updates (Note, NoteFolder, ContentItem)
• note/folder use cases
• content item use cases
• AI-related use cases
• package/build cleanup

The reason I suggest this is not that the work is wrong. It is just hard to review and debug when architectural changes, new logic, tests, and build/config updates all come together in one PR.

Smaller PRs would make it easier to:

• review each idea separately
• understand which change caused a CI/test problem
• give more focused feedback
• merge safe parts earlier

One possible split could be:

1. domain model changes
2. note/folder use cases
3. content item use cases
4. AI use cases
5. build/package cleanup

It does not have to be exactly this split, but separating the bigger architectural changes from the feature logic would already make the review much easier.