feat(desktop): now playing with playback controls and queue (P3-11)

[forkwright]

Summary

• Add full now-playing bar (transport controls, progress bar, volume, quality indicator) to the desktop app's fixed bottom area
• Implement Rust Tauri playback backend: DesktopQueue with shuffle/repeat, stream download via HTTP, akroasis-core Engine integration, 18 IPC commands
• Add features/now-playing/ TypeScript feature: Zustand store, hooks (usePlaybackState, useQueue, useSignalPath, useKeyboardShortcuts), components (NowPlayingBar, TransportControls, ProgressBar, VolumeControl, TrackInfo, QualityIndicator, ExpandedView), pages (QueuePage, SignalPathPage)
• Wire library integration: double-click-to-play and per-row play/add-to-queue on TracksPage; play-album/shuffle-album/add-to-queue overlays on AlbumsPage
• Add /queue and /signal-path routes; keyboard shortcuts (Space, arrows, M, N, P, S, R)

Blast radius

• desktop/src-tauri/Cargo.toml — new deps: snafu, tracing, rand, akroasis-core, tokio sync/time
• desktop/src-tauri/src/lib.rs — registers 18 new playback IPC commands
• desktop/src-tauri/src/playback/ — new module: mod.rs, queue.rs, stream.rs, signal_path.rs, commands.rs
• desktop/src/types/playback.ts — new shared TS types
• desktop/src/features/now-playing/ — new feature directory (store, hooks, components, pages)
• desktop/src/components/Layout.tsx — replaces placeholder with <NowPlayingBar />
• desktop/src/App.tsx — adds /queue and /signal-path routes
• desktop/src/features/library/TracksPage.tsx — double-click-to-play, play/add-to-queue buttons
• desktop/src/features/library/AlbumsPage.tsx — album play/shuffle/add-to-queue overlays
• desktop/src/api/client.ts — adds listTracksForAlbum

Validation gate

All gates pass:

• cargo check — zero errors
• cargo clippy -- -D warnings — zero warnings
• npm run lint (eslint) — zero errors
• npm run build (tsc + vite) — clean build, 382 KB bundle

Test plan

• Build desktop app with npm run tauri dev
• Verify now-playing bar renders at bottom of layout
• Verify transport controls (play/pause/next/prev/shuffle/repeat) invoke IPC
• Verify progress bar click-to-seek works
• Verify double-click on a track row in Tracks tab starts playback
• Verify ▶ and + buttons appear on track row hover
• Verify album card shows play/shuffle/add overlays on hover
• Verify /queue route renders drag-to-reorder queue page
• Verify /signal-path route renders signal path chain
• Verify keyboard shortcuts: Space (play/pause), N/P (next/prev), S (shuffle), R (repeat), M (mute)
• Verify expanded view opens/closes via button and Escape key

Observations

• listTracksForAlbum calls /api/music/release-groups/{id}/tracks; this endpoint is not yet in Mouseion. A follow-up backend PR is needed. Album play actions fail gracefully (empty tracks → no-op) until then.
• akroasis-core Engine requires unsafe impl Send + Sync for PlaybackEngine because Arc<Engine> cannot be proven Send automatically; Engine declares these traits explicitly via unsafe impl in the core crate.
• Stream download fetches the entire audio body before writing to a temp file. For large lossless files this is a concern. Chunked streaming write is a future optimization.

[github-actions]

⚠️ Large PR detected — 26 files, 3088 lines changed.

Consider splitting into smaller PRs for easier review. Not a blocker, just a signal.