Fix alphabet highlight for names starting with "The " (#1788)

## Description
The alphabet scroll bar highlighted the letter of an item's *display*
name while the grid sorted by the server's article-stripped `SortName`,
so "The Beatles" sat under **B** but highlighted **T**.

- **Artists:** the Artists tab never requested `SortName`, so the
highlight silently fell back to `Name`. It now asks for `SlimItemFields`
(which already includes `SORT_NAME`), matching the sibling `/Items`
branch. The count-only query used for letter positions passes `fields =
null`, since a `limit = 0` request has no use for them.
- **Genres and studios:** these grids bucketed on the display name
unconditionally. The sort-name fallback is extracted as
`BaseItemDto.alphabetSortName` (`sortName ?: name ?: ""`) and carried
onto the `Genre` and `Studio` grid items, so all three grids now derive
the letter from one value.

### Related issues
https://github.com/damontecres/Wholphin/issues/1787

### Testing
Two unit tests added:
- `CollectionFolderViewModelTest` pins that the artist pager actually
requests `SORT_NAME` — the field whose absence caused the bug.
- `AlphabetSortNameTest` covers both branches of the `alphabetSortName`
fallback.

Manually: select a music library → Artists tab → put the cursor on an
artist starting with "The " and confirm the highlighted letter matches
the grid position. Same check on the genre and studio grids.

## AI or LLM usage
I used Cursor for debugging and implementing the fix, and Claude Code
for a follow-up cleanup pass over the diff.

app/src/main/java/com/github/damontecres/wholphin/data/model/BaseItem.kt

@@ -54,7 +54,7 @@ data class BaseItem(
         get() = type.playable
 
     override val sortName: String
-        get() = data.sortName ?: data.name ?: ""
+        get() = data.alphabetSortName
 
     val type get() = data.type
 
@@ -305,6 +305,9 @@ data class BaseItem(
 
 val BaseItemDto.aspectRatioFloat: Float? get() = width?.let { w -> height?.let { h -> w.toFloat() / h.toFloat() } }
 
+/** The server strips leading articles (eg "The ") from [BaseItemDto.sortName]. */
+val BaseItemDto.alphabetSortName: String get() = sortName ?: name ?: ""
+
 @Immutable
 data class BaseItemUi(
     val episodeCornerText: String?,