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.

tests/test_app.py

@@ -157,3 +157,23 @@ async def test_stage_shows_bandcamp_link_only_when_present() -> None:
         app.query_one(StagePanel).refresh_stage()
         await pilot.pause()
         assert "Bandcamp" not in rendered_text(app)
+
+
+@pytest.mark.asyncio
+async def test_media_session_commands_drive_the_app() -> None:
+    app = SomaFMApp(autoplay=False, offline=True, audio=False)
+    async with app.run_test(size=(100, 30)) as pilot:
+        await pilot.pause()
+        # The status bar names whichever backend this platform uses.
+        assert app.media.label in {"mpris", "nowplaying"}
+        assert app.media.status_label == "off"  # audio disabled
+
+        # The ticker drains the queue, so give it a couple of ticks.
+        before = app.current.id
+        app.media_actions.put("next")
+        await pilot.pause(0.2)
+        assert app.current.id != before
+
+        app.media_actions.put("volume:25")
+        await pilot.pause(0.2)
+        assert app.volume == 25