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/test/java/com/github/damontecres/wholphin/data/model/AlphabetSortNameTest.kt

@@ -0,0 +1,36 @@
+package com.github.damontecres.wholphin.data.model
+
+import org.jellyfin.sdk.model.api.BaseItemDto
+import org.jellyfin.sdk.model.api.BaseItemKind
+import org.junit.Assert.assertEquals
+import org.junit.Test
+import java.util.UUID
+
+class AlphabetSortNameTest {
+    @Test
+    fun prefersSortNameOverName() {
+        val dto =
+            BaseItemDto(
+                id = UUID.randomUUID(),
+                type = BaseItemKind.MUSIC_ARTIST,
+                name = "The Beatles",
+                sortName = "beatles",
+            )
+
+        assertEquals("beatles", dto.alphabetSortName)
+        assertEquals("beatles", BaseItem(dto).sortName)
+    }
+
+    @Test
+    fun fallsBackToNameWhenSortNameMissing() {
+        val dto =
+            BaseItemDto(
+                id = UUID.randomUUID(),
+                type = BaseItemKind.STUDIO,
+                name = "The Walt Disney Company",
+                sortName = null,
+            )
+
+        assertEquals("The Walt Disney Company", dto.alphabetSortName)
+    }
+}

app/src/test/java/com/github/damontecres/wholphin/ui/components/CollectionFolderViewModelTest.kt

@@ -33,10 +33,12 @@ import org.jellyfin.sdk.api.operations.UserLibraryApi
 import org.jellyfin.sdk.model.api.BaseItemDto
 import org.jellyfin.sdk.model.api.BaseItemKind
 import org.jellyfin.sdk.model.api.CollectionType
+import org.jellyfin.sdk.model.api.ItemFields
 import org.jellyfin.sdk.model.api.request.GetArtistsRequest
 import org.jellyfin.sdk.model.api.request.GetItemsRequest
 import org.junit.After
 import org.junit.Assert.assertEquals
+import org.junit.Assert.assertTrue
 import org.junit.Before
 import org.junit.Test
 import java.util.UUID
@@ -141,6 +143,19 @@ class CollectionFolderViewModelTest {
             coVerify(exactly = 0) { GetItemsRequestHandler.execute(any(), any<GetItemsRequest>()) }
         }
 
+    /**
+     * Without SortName, the alphabet highlight falls back to Name and highlights "T" for artists
+     * like "The Beatles" even though the grid sorts them under "B".
+     */
+    @Test
+    fun `artist pager requests SortName for alphabet highlighting`() =
+        runTest(testDispatcher) {
+            createViewModel(GetItemsFilter(override = GetItemsFilterOverride.ARTIST))
+            advanceUntilIdle()
+
+            assertTrue(ItemFields.SORT_NAME in artistsRequest.captured.fields.orEmpty())
+        }
+
     /**
      * /Persons has no nameLessThan or startIndex, so counting up to a letter is not expressible.
      * Falling back to /Items would count media rather than people, ie the same defect as the artist