Entity refactoring: normalized state store with service layer

[Copilot]

Implements normalized state management architecture separating data models (entities), state container (AppStore), mutation logic (services), and presentation (views).

Architecture

Entities (Store/Entities.swift)

• Unified models: Track, Album, Artist, Playlist, Device
• Sendable, ID-based relationships, no nested objects
• Denormalized display fields (artistName, albumName, imageURL) to avoid lookup overhead

AppStore (Store/AppStore.swift)

• Normalized entity dictionaries keyed by ID
• ID arrays for ordered collections (userPlaylistIds, savedTrackIds)
• Set-based favorites for O(1) lookup (favoriteTrackIds)
• Computed properties return entity arrays from IDs

Services (Store/Services/)

• Handle API calls, convert responses to entities, update AppStore
• Task deduplication in Album/Artist/Playlist services (prevents race conditions during 2→3 column layout transitions)
• Optimistic updates with rollback (e.g., favorite toggling)

Usage Pattern

struct AlbumDetailView: View {
    @Environment(AppStore.self) private var store
    @Environment(AlbumService.self) private var albumService
    
    private var tracks: [Track] {
        store.albums[albumId]?.trackIds.compactMap { store.tracks[$0] } ?? []
    }
    
    func loadTracks() async {
        _ = try? await albumService.getAlbumTracks(albumId: id, accessToken: token)
        // Tracks now in store.tracks, view updates automatically
    }
}

Service Initialization

Services with task deduplication or subscriptions stored as @State in LoggedInView to survive view recreation. Stateless services created on-demand—all state lives in AppStore.

Conversions

API response types (APITrack, APIAlbum) separated from entities. Extension-based conversions in EntityConversions.swift handle context-aware initialization (e.g., tracks from album endpoint lack album metadata).

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[ralph]

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