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.

README.md

@@ -285,37 +285,44 @@ All connected WebSocket clients receive a `links_changed` notification when link
 
 ## Installation
 
+Dependencies are declared in `pyproject.toml`. Pick the extra that matches
+your OS — the Python deps differ enough between Windows / Linux / macOS
+that there's no single `pip install` line.
+
 ### Installing on Windows
 
 ```bash
-pip install -r requirements.txt
+pip install ".[windows]"
 ```
 
-Required packages: `winsdk`, `pywin32`, `pycaw`, `comtypes`
+Pulls in `winsdk`, `pywin32`, `pycaw`, `comtypes`, `pystray`, etc.
 
 ### Installing on Linux
 
 ```bash
-# Install system dependencies
-sudo apt-get install python3-dbus python3-gi libdbus-1-dev libglib2.0-dev
+# System packages required to build dbus-python + PyGObject from sdist.
+sudo apt-get install -y python3-pip python3-venv \
+    libdbus-1-dev libglib2.0-dev pkg-config
 
-pip install -r requirements.txt
+pip install ".[linux]"
 ```
 
 ### Installing on macOS
 
 ```bash
-pip install -r requirements.txt
+pip install ".[macos]"
 ```
 
-No additional dependencies - uses built-in `osascript`.
+Pulls in `pyobjc-framework-Cocoa` + `pyobjc-framework-Quartz` for the
+foreground-window probe; AppleScript-based media control uses the
+built-in `osascript`.
 
 ### Installing on Android (Termux)
 
 ```bash
 # In Termux
 pkg install python termux-api
-pip install -r requirements.txt
+pip install "."
 ```
 
 Requires Termux and Termux:API apps from F-Droid.
@@ -835,11 +842,19 @@ Install:
 sudo ./service/install_linux.sh install
 ```
 
-Enable and start for your user:
+**Enable user lingering** — required so `/run/user/$UID/bus` (the D-Bus
+session socket needed for MPRIS) exists even when no graphical session
+is active. Without this the server boots but every `/api/media/*` call
+silently returns idle.
 
 ```bash
-sudo systemctl enable media-server@$USER
-sudo systemctl start media-server@$USER
+sudo loginctl enable-linger $USER
+```
+
+Enable and start the templated unit for your user:
+
+```bash
+sudo systemctl enable --now media-server@$USER
 ```
 
 View logs:
@@ -848,6 +863,39 @@ View logs:
 journalctl -u media-server@$USER -f
 ```
 
+**Troubleshooting:**
+
+- *"`/api/media/status` always returns `idle`"* — check the service log for
+  `D-Bus session bus not available`. Most commonly: lingering isn't
+  enabled, or the unit is using the wrong `XDG_RUNTIME_DIR` (`%U` must
+  expand to the user's numeric UID).
+- *"Visualizer permanently unavailable"* — PulseAudio/PipeWire must
+  expose monitor sources. `pactl list sources short | grep monitor`
+  should list at least one entry; if not, install `pipewire-pulse` and
+  restart your session.
+- *"Volume control silently fails"* — `pactl` must be on `PATH` and the
+  user's PulseAudio/PipeWire server must be reachable
+  (`PULSE_RUNTIME_PATH=/run/user/$UID/pulse`).
+- *"Foreground window is always `null`"* — expected under Wayland; the
+  compositor hides window info from unprivileged clients. X11 sessions
+  work normally.
+
+### macOS (LaunchAgent)
+
+The distribution tarball ships an installer:
+
+```bash
+./install-launchagent.sh
+```
+
+This drops `~/Library/LaunchAgents/com.dolgolyov.media-server.plist`,
+starts the service immediately, and re-launches it at every login. Logs
+go to `~/Library/Logs/media-server/{stdout,stderr}.log`. To stop:
+
+```bash
+./uninstall-launchagent.sh
+```
+
 ## Command Line Options
 
 ```text