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.toml

@@ -47,6 +47,9 @@ thread-priority = "1"
 lofty = "0.22"
 id3 = "1.16.4"
 
+[target.'cfg(unix)'.dependencies]
+libc = "0.2"
+
 [target.'cfg(windows)'.dependencies]
 windows = { version = "0.58", features = [
     "Win32_Foundation",