feat(subsonic): Sorting like file explorers

[ummshsh]

Good day!

I thought it makes sense to have option to see folders sorted just like you see them in file explorers. Certainly makes my live easier =)

Problem

Gonic sorts folder listings by right_path COLLATE NOCASE — raw bytewise comparison. This means ( sorts before _ (ASCII 40 vs 95).
File explorers (Finder, Explorer, Nautilus) use the Unicode Collation Algorithm, which gives _ a lower weight than ( — so _1 comes before (1992) One.

Solution

Pre-computed sort key column — same approach file explorers use under the hood.
linguistic (UCA-based) folder sorting with admin toggle

What changed (10 files)

• db/db.go — Added RightPathSortKey field to Album model + new LinguisticSorting setting key
• db/migrations.go — Migration 202607051200 adds the column via AutoMigrate and backfills sort keys for all existing albums using golang.org/x/text/collate
• scanner/scanner.go — populateAlbumBasics always writes a UCA sort key from the directory basename during scan
• server/ctrlsubsonic/ctrl.go — Three helper methods (sortRightPath, sortRightPathPrefixed, sortRightPathParentPrefixed) that check the linguistic_sorting setting and return either right_path_sort_key or right_path COLLATE NOCASE
• server/ctrlsubsonic/handlers_by_folder.go — All 6 ORDER BY clauses switched to use helpers
• server/ctrlsubsonic/handlers_by_tags.go — Tag-mode artist album listing switched to use helper
• server/ctrladmin/handlers.go — Loads setting in ServeHome, added POST handler to save toggle
• server/ctrladmin/ctrl.go — Registered route, added LinguisticSorting to template data
• server/ctrladmin/adminui/pages/home.tmpl — New "sorting" block with dropdown (bytewise / linguistic)
• go.mod — Promoted golang.org/x/text to direct dependency

How it works

The sort key is always written during scanning regardless of the toggle. The toggle (/admin/home → sorting dropdown) only changes which column the ORDER BY uses at query time — zero-rebuild, instant switch. Migration backfills existing albums so there's no gap.

Migration behavior

On upgrade, migrateAlbumRightPathSortKey:

1. Adds right_path_sort_key TEXT column via AutoMigrate
2. Iterates all albums and sets the sort key from right_path using UCA
3. No downtime — column is nullable, next scan fills any newly added albums

[ummshsh]

This approach targets only folders, I don't know if it makes sense to do the same for individual tracks.