feat: production-ready Linux & macOS support

- Add `linux` (dbus-python, PyGObject, python-xlib) and `macos`
  (pyobjc) extras to pyproject.toml with sys_platform markers; move
  cross-platform screen-brightness-control + monitorcontrol to base deps.
- build-dist-linux.sh: install `.[linux]`, pkg-config pre-flight for
  dbus-1/glib-2.0, emit a systemd unit with DBUS_SESSION_BUS_ADDRESS +
  XDG_RUNTIME_DIR + ReadWritePaths for ~/.config and ~/.cache so MPRIS
  works and audit-log / thumbnail writes aren't blocked by ProtectHome.
- New build-dist-macos.sh + per-user LaunchAgent installer producing
  MediaServer-vX.Y-macos-{arm64,x86_64}.tar.gz.
- Templated media-server.service updated to match the dist layout with
  proper session-bus env vars and a writable state-dir grant.
- install_linux.sh: drop dead requirements.txt path; install via
  `pip install ".[linux]"` and pre-create the writable state dirs.
- Cross-platform album artwork: abstract MediaController.get_album_art()
  with Linux (mpris:artUrl, file:// + http(s)://) and macOS (Spotify URL)
  impls; routes/media artwork endpoint now awaits the controller.
- LinuxMediaController connects to the session bus lazily — failure no
  longer crashes lifespan startup; MPRIS calls return idle until the bus
  is reachable. Logged once at INFO with a hint about
  `loginctl enable-linger`.
- Startup preflight on Linux warns if DBUS_SESSION_BUS_ADDRESS or
  XDG_RUNTIME_DIR is unset and informs the user when Wayland disables
  the foreground probe.
- /api/media/visualizer/status now reports a per-OS unavailable_reason.
- tray._confirm guarded against ctypes.windll on non-Windows.
- config.example.yaml: per-OS commented script examples; on_turn_off
  default is now a no-op echo (used to silently fail off Windows).
- README: replace stale `pip install -r requirements.txt` instructions
  with the new extras; add systemd lingering doc + troubleshooting
  section; add macOS LaunchAgent section.
- CI: new linux-smoke job (installs `.[linux]`, boots the server under
  dbus-run-session, asserts /api/health). Release workflow gains
  apt-deps step for the Linux build and a best-effort macOS build job.

media_server/services/__init__.py

@@ -65,7 +65,12 @@ def get_media_controller() -> "MediaController":
 
 
 def get_current_album_art() -> bytes | None:
-    """Get the current album art bytes (Windows only for now)."""
+    """Get the current album art bytes (synchronous, Windows-cached path).
+
+    Windows pre-populates a module-level cache via the WinRT polling thread,
+    so this stays sync. For Linux/macOS the controller fetches on demand —
+    use ``get_current_album_art_async()`` from FastAPI handlers instead.
+    """
     system = platform.system()
     if system == "Windows":
         from .windows_media import get_current_album_art as _get_art
@@ -73,6 +78,22 @@ def get_current_album_art() -> bytes | None:
     return None
 
 
+async def get_current_album_art_async() -> bytes | None:
+    """Cross-platform album art fetch. Awaits the controller's impl.
+
+    Falls back to the sync Windows cache when running on Windows so we don't
+    pay an extra coroutine hop for the existing path.
+    """
+    system = platform.system()
+    if system == "Windows":
+        return get_current_album_art()
+    controller = get_media_controller()
+    try:
+        return await controller.get_album_art()
+    except Exception:  # noqa: BLE001 — art is best-effort; never break the route
+        return None
+
+
 def get_audio_devices() -> list[dict[str, str]]:
     """Get list of available audio output devices (Windows only for now)."""
     system = platform.system()
@@ -82,4 +103,9 @@ def get_audio_devices() -> list[dict[str, str]]:
     return []
 
 
-__all__ = ["get_media_controller", "get_current_album_art", "get_audio_devices"]
+__all__ = [
+    "get_media_controller",
+    "get_current_album_art",
+    "get_current_album_art_async",
+    "get_audio_devices",
+]