Restrict ViewModel visibility to `internal` within feature modules

## Summary

All `ViewModel` classes across the project are currently declared as `public`, even though they are only used within their respective feature modules. This can lead to unintended usages across modules and violates encapsulation principles.

## Proposal

Change the visibility modifier of each `ViewModel` from `public` to `internal` **if it is not used outside the module** it belongs to.

This adjustment:

- Prevents accidental access from other modules
- Reinforces module boundaries
- Keeps internal logic private to its domain
- Makes the codebase more maintainable and intention-revealing

### Example

```kotlin
// Current (public by default)
@HiltViewModel
class BookmarksViewModel @Inject constructor(...) : ViewModel() {
...
}

// Suggested
@HiltViewModel
internal class BookmarksViewModel @Inject constructor(...) : ViewModel() {
...
}

Search the project for @HiltViewModel classes and public ViewModel declarations, starting with the BookmarksViewModel example. Inspect each ViewModel's references across feature modules before changing visibility; done means only ViewModels unused outside their own module are internal and the project still compiles.

Written by the indexing model from the issue text.