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.

pyproject.toml

@@ -1,7 +1,7 @@
 [project]
 name = "somafm-tui"
 version = "0.1.0"
-description = "A flashy terminal radio for SomaFM with a live spectrum analyser, MPRIS support and song history"
+description = "A flashy terminal radio for SomaFM with a live spectrum analyser, MPRIS / macOS Now Playing support and song history"
 readme = "README.md"
 license = "WTFPL"
 license-files = ["LICENSE"]
@@ -17,6 +17,9 @@ classifiers = [
   "Programming Language :: Python :: 3.12",
   "Programming Language :: Python :: 3.13",
   "Topic :: Multimedia :: Sound/Audio",
+  "Operating System :: POSIX :: Linux",
+  "Operating System :: MacOS",
+  "Environment :: MacOS X",
 ]
 dependencies = [
   "textual>=8.0,<9",