fix(api): request SortName field so fast scroll matches the sorted list

[Thomas05000005]

Summary

Fixes the fast scroll alphabet jumping to the wrong letter for items whose display name and sort name differ. The bug reported in the issue (and reproduced by @DanielDeur in the linked video) is "click L → jumps to Les Bains Douches even though the album is correctly sorted under B because its sort title is Bains Douches, Les".

Closes #978

Root cause

music_screen_tab_view.dart's scrollToLetter already uses BaseItemDto.nameForSorting, and nameForSorting already prefers sortName over name when available. The actual problem is that sortName was never being filled in by Jellyfin in the first place: the defaultFields constant in jellyfin_api.dart:21 didn't list SortName, so the Items API never returned it. With sortName == null, nameForSorting falls back to name (and the English-only "the / a / an" stripping), which doesn't match the server-side sort order for non-English titles like the "Les …" example, items with forcedSortName, or any user-customized sort key.

The fix

Add SortName to defaultFields. That's the only change. nameForSorting already does the right thing once sortName is populated.

-const String defaultFields = "…,ProviderIds,Tags";
+const String defaultFields = "…,ProviderIds,Tags,SortName";

SortName is documented as a valid Fields value directly inside this same file (the inline docstring on the getItems @Query("Fields") parameter lists it), so this isn't introducing anything that needs a Jellyfin server-version gate.

Why this is safe

• The change is purely additive: existing callers that pass an explicit fields argument are unaffected, and no field is removed.
• Cost: one extra short string per item in the API response. Negligible vs. the existing payload (covers, tags, blurhash, etc.).
• No new code path: nameForSorting already handles sortName correctly; we're just feeding it data.
• Touches no Hive schema and adds no setting, so there's no migration concern.

flutter analyze returns 0 new issues. Code formatted with dart format.

Test plan

⚠️ Transparency: Like #1615, #1616, #1617, #1618, this PR is opened as a draft because I have not been able to run the app on a physical device yet. Help testing it on a device would be very welcome.

Manual checks to perform on a device (a library with at least one album whose name starts with a different letter than its sortName is needed - French, Spanish, German articles, or any album with a custom forcedSortName works):

• Open the music tab, sort by "Sort name"
• Verify the album sits in the list at the letter of its sort name (e.g. Les Bains Douches under B)
• Tap the B letter on the right-hand fast scroller - the list should now jump to that section
• Tap the L letter - should NOT jump to Les Bains Douches anymore (it should land on actual L* items)
• Same checks while sorted by "Album artist"
• Reinstall: first-load albums also have sortName populated (this would catch any cache invalidation oversight, but I don't believe one is needed here)

[Thomas05000005]

Hi,

Quick clarification, since #1623 was merged recently:

• Fix(fast scroll)/jump to letter when ordered by album artist #1623 (merged) resolves issue Alphabetic sidebar weird when albums ordered by artist #1609: client-side sorting of multiple album-artists and stripping of diacritics for fast-scroll.
• This PR (fix(api): request SortName field so fast scroll matches the sorted list #1619) resolves issue Fast scroll letters should use the sort by field #978: asking the server to actually return the sortName field (one extra entry in defaultFields) so that nameForSorting can use it.

Without #1619, an album like "Les Bains Douches" keeps being sorted incorrectly because its server-side sortName ("Bains Douches, Les") is never included in the API response. nameForSorting then falls back to "name + the/a/an English stripping", which doesn't cover French/Spanish/German articles or custom forcedSortName values.

The two changes don't overlap (different files and code paths) and remain complementary.

Thanks!