feat: sort playlist tracks by title, artist, or duration

[tsibog]

Description

Adds a sort control to the playlist detail page. Tracks can be sorted by Title, Artist, or Duration (selecting the active option again toggles ascending/descending), with Custom order (the playlist's own order) as the default. The sorted order drives everything consistently: the visible list, row taps, and the header Play / Play Next / Add to Queue actions. Sorting is session-only view state — it never mutates the playlist.

"Date added" was considered and dropped: the browse response for VL<playlistId> carries no per-track added date and the API exposes no server-side sort for playlist contents (only library-level listings support an order param).

AI Prompt (Optional)

🤖 AI Prompt Used

I want to add sorting to the playlist detail view: by artist name, title,
date added (if possible) and duration. Look around in this repo for
guidelines and various docs.

AI Tool: Claude Code

Type of Change

• ✨ New feature (non-breaking change that adds functionality)

Related Issues

None found.

Changes Made

• PlaylistTrackSortOption + sortOption/sortAscending/displayedTracks/selectSort(_:) on PlaylistDetailViewModel — all sorting logic lives behind this small surface (localized standard compare, artist ties broken by title, nil durations always last, direction toggle on re-select)
• PlaylistDetailView+TrackSort.swift — compact trailing sort Menu above the track list (checkmark + direction chevron on the active option), hidden for playlists with fewer than 2 tracks
• PlaylistDetailView / +HeaderActions consume displayedTracks so display and playback order always match; the deferred full-playlist load now diffs queued tracks by videoId instead of a fixed count so a re-sort during pagination can't drop or duplicate tracks

Testing

• Unit tests pass (swift test --skip KasetUITests)
• Manual testing performed — sorted playlists by each option in both directions, played from sorted views, verified header actions follow the sorted order and Custom restores the original order
• UI tested on macOS 26+

Checklist

• My code follows the project's style guidelines
• I have run swiftlint --strict && swiftformat .
• I have added tests that prove my fix/feature works (5 new PlaylistDetailViewModelTests covering each option, direction toggle, artist tiebreak, nil-duration ordering)
• New and existing unit tests pass locally
• I have updated documentation if needed
• I have checked for any performance implications
• My changes generate no new warnings

Screenshots

Additional Notes

Includes the same test: deflake commit as #363/#364 (the NotificationServiceTests fixed-wait flake lives on main and hits fresh branches); it disappears on rebase once any of them merges.

Agent Transcript

Redacted agent session transcript (scoped to this PR)

source: local Claude Code session, trimmed to this PR's scope
omitted: raw tool outputs, system prompts, local paths, unrelated session turns

[user]
so i want to add sorting here [screenshot of playlist detail], by artist name,
title, date added (if possible) and duration. look around in this repo for
guidelines and various docs.

[assistant]
Scouted the repo first. Findings: the playlist page is PlaylistDetailView +
PlaylistDetailViewModel (@Observable), tracks are [Song] with title/artists/
duration all available client-side. No "date added" exists anywhere in the
parsed API data. No sort UI precedent in the app. The play queue is built from
the displayed array, so a sorted view plays in sorted order for free; row
removal keys off setVideoId, not index, so it's safe under re-sorting.

[user]
(decision) Investigate the API before dropping "date added"; sort state should
be session-only, not persisted.

[assistant]
Investigated: no per-track added date in the browse response for VL<playlistId>,
and no server-side sort params exist for playlist contents (only library-level
listings have an `order` param; cross-checked against ytmusicapi's parsers and
API surface). Verdict: date added is not feasible — shipping Custom order /
Title / Artist / Duration.

Implementation: PlaylistTrackSortOption + selectSort()/displayedTracks on the
view model (localizedStandardCompare, artist ties broken by title, nil
durations last, re-select toggles direction); compact sort Menu above the
track list; all consumers (rows, Play, Play Next, Add to Queue) switch to
displayedTracks. The deferred full-playlist load now diffs by videoId instead
of a fixed count so a re-sort during pagination can't drop or duplicate tracks.

[assistant]
Verification: swiftlint --strict clean, swiftformat clean, swift build OK,
swift test --skip KasetUITests → 1779/1779 pass incl. 5 new
PlaylistDetailViewModelTests (each option, direction toggle, artist tiebreak,
nil-duration ordering, reset-to-ascending). A separate guideline+architecture
review pass confirmed compliance (no print/force-unwraps, Swift Testing,
localization) and that the tests exercise only the view model's public surface.

[user]
Sorting works.

[tsibog]

Closing in favor of #375. The sort feature works, but placing the sort control in the header action row causes scroll/hover jank (ViewThatFits re-measuring under the ScrollView). Handing the placement/design decision to a maintainer — details, root-cause bisect, and options in #375. Branch pr/playlist-sort retains the implementation + tests for reference.