[iOS & tvOS] FilterViewModel - Cleanup

[JPKribs]

Summary

Resolves: #1246

This PR updates the FilterViewModel to be Stateful. Additionally, all of the filter logic is moved from the FilterView to the ViewModel. Finally, while I was working on the FilterView, I found that #1246 was still present. I've updated the SelectorView and tested this from both the FontPickerView and FilterView and this is now working as expected.

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I feel very good about the FilterViewModel change to Stateful. As I understand it, the data functionality should be tied to the ViewModel instead of the view. Please let me know if that is not the case or if there is any reason this view was intentional left as non-Stateful.

The SelectorView is functional in both locations it's called from but not pretty. To make this functional for both the String and AnyItemFilter I had to have the selection as both a State and a Binding. I'm open to feedback if there is a cleaner way to do this but I was trying to accomplish this without having to change the callers too dramatically.

If we decide the SelectorView changes are a wash, I'm perfectly fine with this PR focusing on the FilterViewModel changes instead! Let me know!

Final item, the SearchViewModel didn't have a filter for Letters. I've added that as a drop in copy/paste from the LibraryItemViewModel

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Before:

The reset appropriately resets the filter but the UI does not update. This is because the selection in a binding box is not updating correctly. My solution was to have a State item for editing that updated the Binding item on change.

Simulator.Screen.Recording.-.iPhone.16.Pro.-.2025-01-27.at.18.51.07.mp4

After:

Simulator.Screen.Recording.-.iPhone.16.Pro.-.2025-01-27.at.18.44.13.mp4

[JPKribs]

This one should be ready to go. The only item that I was considering adding but didn't was filtering the filters. Sometimes, there is a navigationFilterBar when there are no options for that filter.

A good example of this Favorites. You can look at the favorites but there is no option for Genres or Filters in the dropdown. I would argue if the FilterViewModel returns nothing in AllFilters that dropdown should be hidden.

[LePips]

While it is nice that things are moved out of FilterView and makes FilterViewModel easier to use, FilterViewModel doesn't really need to be fully Stateful - it's only really meant as an observable source of filter information. The only thing that would be "stateful" would be getting the query filters for the library parent, but that can kind of be hidden away from the user in this case.

I'll have to think about the overall architecture of this.