feat(macos): drive the system Now Playing session through mpv

macOS has no D-Bus/MPRIS: the audio engine is what the OS media UI talks
to. mpv's Cocoa backend registers with MPRemoteCommandCenter and
MPNowPlayingInfoCenter, so it already publishes the live ICY title and
receives the media keys.

Add a platform-neutral media session (mediasession.py) so the app keeps a
single interface: MprisService on D-Bus platforms, NowPlayingSession on
macOS. Map mpv's NEXT/PREV/PLAY/PLAYONLY/PAUSEONLY/STOP keys onto
script-messages and read them back as client-message IPC events, so the
hardware media keys and Control Center drive the TUI (station changes,
play/pause/stop) instead of fighting it.

--no-mpris (now also --no-media) disables the media keys too. macOS-only
mpv options are gated on sys.platform so the Linux command line is
unchanged; mpv aborts on unknown flags.

somafm_tui/player.py

@@ -20,6 +20,7 @@ import shlex
 import shutil
 import signal
 import subprocess
+import sys
 import threading
 import time
 from collections.abc import Callable
@@ -28,12 +29,35 @@ from pathlib import Path
 from typing import Any
 
 from .api import USER_AGENT
-from .config import RUNTIME_DIR
+from .config import RUNTIME_DIR, atomic_write_text
+
+#: mpv input bindings that turn the system media keys into TUI commands.
+#:
+#: On macOS mpv's Cocoa backend implements MPRemoteCommandCenter, so the
+#: hardware/keyboard media keys arrive as these special key names.  Instead of
+#: letting mpv handle them (which would fight the TUI and desync the station
+#: list, the spectrum and the desktop "now playing" state) we bounce them back
+#: over the IPC socket as ``client-message`` events.
+MEDIA_KEY_BINDINGS = """\
+# Generated by somafm-tui -- do not edit; it is rewritten on every launch.
+PLAY script-message somafm toggle
+PLAYONLY script-message somafm play
+PAUSEONLY script-message somafm pause
+STOP script-message somafm stop
+NEXT script-message somafm next
+PREV script-message somafm prev
+"""
+
+#: Commands a media key may ask for; everything else is ignored.
+MEDIA_COMMANDS = frozenset({"play", "pause", "toggle", "stop", "next", "prev"})
+
+#: Prefix of the ``client-message`` namespace we listen on.
+MEDIA_MESSAGE_PREFIX = "somafm"
 
 
 @dataclass
 class PlayerEvent:
-    kind: str = "state"  # "state" | "volume" | "notice"
+    kind: str = "state"  # "state" | "volume" | "notice" | "media"
     state: str = ""
     detail: str = ""
     volume: int | None = None
@@ -41,8 +65,9 @@ class PlayerEvent:
 
 
 class Player:
-    def __init__(self) -> None:
+    def __init__(self, media_keys: bool = True) -> None:
         self.events: queue.Queue[PlayerEvent] = queue.Queue()
+        self.media_keys = media_keys
         self.backend: str | None = None
         self._loop: asyncio.AbstractEventLoop | None = None
         self._thread: threading.Thread | None = None
@@ -295,6 +320,7 @@ class Player:
             f"--user-agent={USER_AGENT}",
             "--network-timeout=20",
         ]
+        command.extend(self._platform_options())
         extra = os.environ.get("SOMAFM_MPV_ARGS")
         if extra:
             command.extend(shlex.split(extra))
@@ -330,6 +356,32 @@ class Player:
         if self._urls:
             await self._mpv_load()
 
+    def _platform_options(self, platform: str | None = None) -> list[str]:
+        """mpv options that only exist (or only make sense) on some platforms.
+
+        Keeping them here means the Linux command line stays exactly what it
+        always was -- mpv aborts on unknown options, so never pass a macOS-only
+        flag unconditionally.
+        """
+        system = sys.platform if platform is None else platform
+        if system != "darwin":
+            return []
+        return self._macos_media_key_options()
+
+    def _macos_media_key_options(self) -> list[str]:
+        """Let mpv publish Now Playing, and route its media keys into the TUI."""
+        if not self.media_keys:
+            return ["--input-media-keys=no"]
+        options = ["--input-media-keys=yes"]
+        try:
+            RUNTIME_DIR.mkdir(parents=True, exist_ok=True)
+            conf = RUNTIME_DIR / "mpv-input.conf"
+            atomic_write_text(conf, MEDIA_KEY_BINDINGS)
+        except OSError:
+            # Now Playing still works; only next/prev/stop stay inside mpv.
+            return options
+        return [*options, f"--input-conf={conf}"]
+
     async def _mpv_send(self, command: list[Any]) -> None:
         writer = self._writer
         if writer is None:
@@ -367,6 +419,10 @@ class Player:
 
     def _handle_mpv_message(self, message: dict[str, Any]) -> None:
         event = message.get("event")
+        if event == "client-message":
+            self._handle_client_message(message.get("args"))
+            return
+
         if event == "property-change":
             name = message.get("name")
             value = message.get("data")
@@ -409,6 +465,20 @@ class Player:
         if error and error != "success":
             self.events.put(PlayerEvent(kind="notice", detail=str(error)))
 
+    def _handle_client_message(self, args: Any) -> None:
+        """Turn ``script-message somafm <command>`` into a player event.
+
+        mpv's macOS backend feeds the system media keys (and Control Center
+        buttons) into its input system; ``MEDIA_KEY_BINDINGS`` maps them onto
+        these messages, which arrive over the JSON IPC socket.
+        """
+        if not isinstance(args, list) or len(args) < 2:
+            return
+        namespace, command = str(args[0]), str(args[1])
+        if namespace != MEDIA_MESSAGE_PREFIX or command not in MEDIA_COMMANDS:
+            return
+        self.events.put(PlayerEvent(kind="media", detail=command))
+
     def _schedule_retry(self, reason: str) -> None:
         if self._retry_scheduled or not self._urls or self._stopping.is_set():
             return