feat: create playlists from the sidebar

[tsibog]

Description

Playlists can now be created from the sidebar: a "+" button reveals on hover over the Playlists section header, opens the existing create-playlist dialog, and on success pins the new playlist to the sidebar and navigates straight to it. Previously the only way to create a playlist was buried in a song's "Add to Playlist" context menu, so an empty account had no visible entry point at all. While creation is in flight the "+" is replaced by a small inline spinner.

The second commit fixes a bug this feature made prominent: deleting a playlist left its sidebar pin behind forever (deletePlaylist never touched SidebarPinnedItemsManager), and the deleted playlist's page stayed open when it had been opened from the sidebar (dismiss() is a no-op there). Deletion is now optimistic — the pin and library entry disappear and navigation happens immediately on confirm; on API failure the pin is restored at its original position, the library entry comes back, and the existing error alert is shown.

AI Prompt (Optional)

🤖 AI Prompt Used

I'd like a little PLUS (+) next to the Playlists label in the left sidebar,
because how do I create a playlist in this app otherwise. Show it only on
hover, with a spinner while the playlist is created. Also deleting a playlist
does nothing visible for seconds and the playlist is stuck in the sidebar —
investigate and lean into optimistic updates.

AI Tool: Claude Code

Type of Change

• ✨ New feature (non-breaking change that adds functionality)
• 🐛 Bug fix (non-breaking change that fixes an issue)

Related Issues

None found.

Changes Made

• Sidebar — custom Playlists section header with a hover-revealed "+" (spinner while creating); the section now renders even with no pinned items so the entry point always exists
• SongActionsHelper.presentCreatePlaylistDialog — the NSAlert create dialog previously inlined in AddToPlaylistContextMenu is extracted so both call sites share one create/broadcast/reconcile path (PlaylistCreationRequest + Result completion)
• LibraryMutationActions.deletePlaylist — optimistic: unpins from the sidebar and broadcasts the library removal before the API call, rolls both back on failure; reconciliation unchanged
• SidebarPinnedItemsManager.insert(_:at:) — position-preserving restore for the rollback path
• onPlaylistDeleted @Entry environment key — the pinned-playlist detail page navigates home after deletion instead of calling dismiss() (a no-op outside a navigation stack); nav-stack contexts keep the dismiss() fallback

Testing

• Unit tests pass (swift test --skip KasetUITests)
• Manual testing performed — created playlists from the sidebar (spinner, pin, navigation), deleted pinned and unpinned playlists, verified instant UI response and Library consistency after reload
• 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 (deletion unpins from sidebar; failed deletion restores pin + library entry)
• 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

kaset-pr370-demo.mp4

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]
i'd like a little PLUS (+) next to the playlists label in the left sidebar,
because how do I create a playlist in this app otherwise.

[assistant]
Scouted first: createPlaylist already exists in the API layer (posts
playlist/create) and an NSAlert create dialog already exists inside the song
context menu — the feature was just undiscoverable. The sidebar Playlists
section is driven by SidebarPinnedItemsManager, which does NOT observe
LibraryMutationBroadcaster, so a created playlist must be explicitly pinned.
Implemented: custom section header with a "+" button, extracted the dialog
into a shared SongActionsHelper.presentCreatePlaylistDialog so both call
sites share one create/broadcast path; on success the playlist is pinned and
opened. Section now renders even with zero pinned items.

[user]
Show the + only when hovering the Playlists header; align its right padding
with the header's left margin.

[user]
Delete playlist looks broken — I confirm the dialog and nothing happens.
...actually it IS deleted (gone from Library), but it's stuck in the sidebar.

[assistant]
Root cause: LibraryMutationActions.deletePlaylist broadcasts the removal to
the Library view models only; nothing ever removes the sidebar pin (pre-
existing bug — any pinned playlist deleted becomes a ghost pin). Also the
detail page stays open because dismiss() is a no-op when the page was opened
from a sidebar pin rather than a navigation stack. Fixed: deletePlaylist now
unpins; a new onPlaylistDeleted @Entry environment value lets the pinned
context navigate home (nav-stack contexts keep the dismiss() fallback).

[user]
Show a spinner (check the app for precedent) while the playlist is created.
Deletion takes seconds before anything happens — lean into optimistic updates.

[assistant]
Precedent check: inline ProgressView().controlSize(.small) is the app's busy
idiom (CommandBar, artist page); no skeleton pattern exists — spinner replaces
the "+" while creating. Deletion made optimistic: pin + library entry removed
and navigation fires immediately on confirm; API call runs after; on failure
the pin is restored at its original index (new SidebarPinnedItemsManager
.insert(_:at:)), the library entry is restored, and the existing error alert
shows.


[assistant]
Verification: swiftlint --strict clean, swiftformat clean, swift build OK,
swift test --skip KasetUITests → 1776/1776 pass incl. 2 new
LibraryMutationActionsTests (deletion unpins from sidebar; failed deletion
restores pin + library entry). Hover animation uses the shared
AppAnimation.quick constant (review-pass finding). Manual test: create from
sidebar (spinner → pin → navigate), delete (instant unpin + redirect home).

[user]
ok, yep, that is good.

[sozercan]

lgtm, thanks

[tsibog]

lgtm, thanks

Happy to contribute to this great project!