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/routes/media.py

@@ -18,7 +18,7 @@ from fastapi.responses import Response
 from ..auth import verify_token, verify_token_or_query
 from ..config import settings
 from ..models import MediaStatus, SeekRequest, VolumeRequest
-from ..services import get_current_album_art, get_media_controller
+from ..services import get_current_album_art_async, get_media_controller
 from ..services.websocket_manager import ws_manager
 
 logger = logging.getLogger(__name__)
@@ -287,7 +287,7 @@ async def get_artwork(
     Returns the bytes with a content-derived ETag so the browser can serve a
     304 when the same track is re-requested.
     """
-    art_bytes = get_current_album_art()
+    art_bytes = await get_current_album_art_async()
     if art_bytes is None:
         raise HTTPException(
             status_code=status.HTTP_404_NOT_FOUND,
@@ -326,14 +326,43 @@ async def get_artwork(
 
 @router.get("/visualizer/status")
 async def visualizer_status(_: str = Depends(verify_token)) -> dict:
-    """Check if audio visualizer is available and running."""
+    """Check if audio visualizer is available and running.
+
+    ``available`` is True only when both numpy + soundcard import.
+    ``unavailable_reason`` carries a short human-readable hint when False
+    or when a usable loopback device was not found — useful on Linux where
+    PulseAudio/PipeWire may not expose monitor sources to a headless
+    systemd-as-user service.
+    """
+    import platform as _platform
+
     from ..services.audio_analyzer import get_audio_analyzer
 
     analyzer = get_audio_analyzer()
+    reason: str | None = None
+    if not analyzer.available:
+        reason = "soundcard or numpy is not installed"
+    elif getattr(analyzer, "_unavailable", False):
+        if _platform.system() == "Linux":
+            reason = (
+                "No loopback audio device found. On Linux this requires a "
+                "running PulseAudio/PipeWire session with monitor sources "
+                "(systemd user service: ensure DBUS_SESSION_BUS_ADDRESS + "
+                "XDG_RUNTIME_DIR are set)."
+            )
+        elif _platform.system() == "Darwin":
+            reason = (
+                "No loopback audio device found. macOS does not expose system "
+                "loopback by default — install BlackHole or Soundflower."
+            )
+        else:
+            reason = "No loopback audio device found"
+
     return {
         "available": analyzer.available,
         "running": analyzer.running,
         "current_device": analyzer.current_device,
+        "unavailable_reason": reason,
     }