feat(desktop): MPRIS, system tray, and OS integration (P3-16)

[forkwright]

Summary

• MPRIS v2 D-Bus service (Linux only, #[cfg(target_os = "linux")]): bridges PlaybackEngine broadcast events to D-Bus PropertiesChanged signals via mpris-server 0.8; supports play/pause/stop/seek/next/previous/volume/shuffle; metadata includes title, artist, album, duration, and track ID
• System tray: icon with context menu (Play/Pause, Next, Previous, now-playing display, Show/Hide window, Quit); left-click toggles window visibility; right-click shows menu; menu rebuilt on each track change via tray.set_menu()
• Desktop notifications: track-change notifications gated behind NotificationConfig flags (enabled, show_artist, show_album); additional helpers for download complete, request approved, and library scan events
• File associations: handle_startup_file checks CLI args at launch for audio paths; handle_file_open emits file-open event consumed by frontend; tauri.conf.json declares fileAssociations for flac/mp3/m4a/m4b/ogg/opus/wav/aac
• Auto-start: tauri-plugin-autostart wired to autostart_is_enabled / autostart_set IPC commands; persisted in AppConfig
• Config persistence: AppConfig (window geometry, tray, notifications, autostart, start-minimized) serialised to ~/.config/harmonia/config.json; window state saved on close/resize/move
• Settings page: General (server URL), System (minimize-to-tray, start-minimized, auto-start), Notifications (enabled + per-field toggles), Audio (link to DSP), About (version/OS/arch)
• NixOS module: programs.harmonia-desktop option; generates xdg.desktopEntries.harmonia, D-Bus activation via services.dbus.packages, and xdg.mime.defaultApplications for all registered audio types

Test plan

• cd desktop && npm run lint — no warnings
• cd desktop && npm run build — clean build
• cd desktop/src-tauri && cargo check — no errors
• cd desktop/src-tauri && cargo clippy -- -D warnings — no warnings
• Launch app; playerctl list shows harmonia
• playerctl play-pause / playerctl next / playerctl previous control playback
• playerctl metadata returns current track title, artist, album
• Media keys on keyboard trigger play/pause/next/previous
• playerctl volume 0.5 sets volume to 50 %
• System tray icon visible; left-click hides/shows window
• Right-click menu shows now-playing line, updates on track change
• Play/Pause/Next/Previous menu items emit correct events
• Track-change notification appears when new track starts
• Notification disabled when notifications.enabled = false
• Settings page opens; all toggles persist across restart
• xdg-open some-file.flac opens Harmonia and emits file-open
• NixOS module syntax: nix-instantiate --parse nix/module.nix

Observations

• desktop/src-tauri/src/playback/podcast.rs and akroasis/shared/akroasis-core/ have uncommitted changes from a prior session that are outside this PR's scope; left unstaged.
• TrayIcon<R> in Tauri 2 exposes no menu() getter — the only update path is rebuilding and calling set_menu(); this means every track change rebuilds the full menu (7 items). Acceptable for now; a MenuItem handle cache would allow targeted set_text() updates.
• mpris-server 0.8 re-exports zbus 4.x as mpris_server::zbus; adding an explicit zbus = "5" dependency creates a type mismatch between zbus::fdo::Error and mpris_server::zbus::fdo::Error. The Cargo.toml must not pin a separate zbus version.
• Album art URL is not yet included in MPRIS metadata (no mpris:artUrl field set). The TrackInfo struct does not carry a cover URL; plumbing that through is a follow-up.

[github-actions]

⚠️ Large PR detected — 24 files, 2387 lines changed.

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