docs(readme): document macOS support, Now Playing and loopback capture

Split the install instructions per platform, add the macOS microphone
permission and BlackHole/Multi-Output recipe for a synced spectrum,
describe the Now Playing integration and its limits, and note that config
and cache stay in the XDG locations. The in-app help text is now
platform-aware too, and the package metadata lists macOS.

somafm_tui/widgets/help.py

@@ -2,6 +2,7 @@
 
 from __future__ import annotations
 
+import sys
 import textwrap
 
 from rich.console import Group
@@ -31,11 +32,14 @@ HELP_ROWS: tuple[tuple[str, str], ...] = (
     ("q", "quit"),
 )
 
-ABOUT = (
+ABOUT_COMMON = (
     (
         "Stations and song history come from somafm.com "
         "(channels.json + songs/<station>.json)."
     ),
+)
+
+ABOUT_LINUX = (
     (
         "The spectrum captures your actual PulseAudio/PipeWire output with "
         "parec (SYNC); otherwise it falls back to a muted ffmpeg stream tap "
@@ -47,6 +51,21 @@ ABOUT = (
     ),
 )
 
+ABOUT_MACOS = (
+    (
+        "The spectrum follows your audio output (SYNC) when a loopback device "
+        "such as BlackHole is available for ffmpeg/avfoundation; otherwise it "
+        "falls back to a muted ffmpeg stream tap (FFT, can drift) or a "
+        "synthesized display (SIM)."
+    ),
+    (
+        "mpv publishes what is playing to the macOS Now Playing session, so "
+        "Control Center and the keyboard media keys control this app."
+    ),
+)
+
+ABOUT = ABOUT_COMMON + (ABOUT_MACOS if sys.platform == "darwin" else ABOUT_LINUX)
+
 
 class HelpScreen(ModalScreen[None]):
     def compose(self) -> ComposeResult: