fix(audio): suppress ALSA stderr noise during device enumeration

Device watcher (3 s loop) and audio_list_devices both called
cpal output_devices(), triggering ALSA to probe unavailable backends
(JACK, OSS, dmix) and spam stderr with error messages.

Fix: redirect fd 2 to /dev/null for the duration of each enumeration
on Unix via libc dup/dup2 + RAII guard. Also add a module-level
cache in Settings.tsx so audio_list_devices is only invoked once
per app session instead of on every Audio tab activation.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src-tauri/Cargo.lock

@@ -3564,6 +3564,7 @@ dependencies = [
  "discord-rich-presence",
  "futures-util",
  "id3",
+ "libc",
  "lofty",
  "md5",
  "reqwest 0.12.28",