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.

pyproject.toml

@@ -34,19 +34,36 @@ dependencies = [
     "pillow>=10.0.0",
     "soundcard>=0.4.0",
     "numpy>=1.24.0,<2.0",
+    # screen-brightness-control + monitorcontrol are cross-platform
+    # (Windows / Linux i2c-dev). Kept in base deps so a plain
+    # `pip install media-server` yields a working /api/display on any
+    # platform that has the underlying hardware support.
+    "screen-brightness-control>=0.20.0",
+    "monitorcontrol>=3.0.0",
 ]
 
 [project.optional-dependencies]
 windows = [
     "winsdk>=1.0.0b10",
-    "pywin32>=306",
-    "comtypes>=1.2.0",
-    "pycaw>=20230407",
-    "screen-brightness-control>=0.20.0",
-    "wmi>=1.5.1",
-    "monitorcontrol>=3.0.0",
+    "pywin32>=306; sys_platform == 'win32'",
+    "comtypes>=1.2.0; sys_platform == 'win32'",
+    "pycaw>=20230407; sys_platform == 'win32'",
+    "wmi>=1.5.1; sys_platform == 'win32'",
     "pystray>=0.19.0",
 ]
+linux = [
+    # MPRIS / D-Bus media control. Requires libdbus-1-dev, libglib2.0-dev,
+    # and pkg-config installed on the host before pip install.
+    "dbus-python>=1.3.2; sys_platform == 'linux'",
+    "PyGObject>=3.46; sys_platform == 'linux'",
+    # X11 foreground-window probe (Wayland sessions degrade gracefully).
+    "python-xlib>=0.33; sys_platform == 'linux'",
+]
+macos = [
+    # PyObjC for the foreground-window + active-app probes.
+    "pyobjc-framework-Cocoa>=10.0; sys_platform == 'darwin'",
+    "pyobjc-framework-Quartz>=10.0; sys_platform == 'darwin'",
+]
 dev = [
     "pytest>=7.0",
     "pytest-asyncio>=0.21",