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/main.py

@@ -112,6 +112,30 @@ async def lifespan(app: FastAPI):
     else:
         logger.warning("No API tokens configured — authentication is DISABLED")
 
+    # Linux preflight: most MPRIS / PulseAudio failures are environmental
+    # (no DBUS_SESSION_BUS_ADDRESS, missing XDG_RUNTIME_DIR, systemd service
+    # started before logind). Surface that early so the failure mode is a
+    # warning at boot instead of silent "/api/media/status returns idle".
+    import os
+    import platform as _platform
+    if _platform.system() == "Linux":
+        missing = [
+            v for v in ("DBUS_SESSION_BUS_ADDRESS", "XDG_RUNTIME_DIR")
+            if not os.environ.get(v)
+        ]
+        if missing:
+            logger.warning(
+                "Linux preflight: %s not set — MPRIS / PulseAudio may be unavailable."
+                " Under systemd, run `loginctl enable-linger <user>` and ensure the"
+                " service unit sets DBUS_SESSION_BUS_ADDRESS + XDG_RUNTIME_DIR.",
+                ", ".join(missing),
+            )
+        if os.environ.get("WAYLAND_DISPLAY"):
+            logger.info(
+                "Wayland session detected — foreground-window probe is intentionally"
+                " disabled (Wayland hides window info from unprivileged clients)."
+            )
+
     update_checker = None
     cleanup_task: asyncio.Task | None = None
     analyzer = None