Fase 2: Add Tauri commands for audio backend system

COMMANDS CREATED (audio_backends.rs):
- get_available_backends() → List backends with availability status
- get_devices_for_backend(type) → Enumerate devices for selected backend
- get_alsa_plugins() → List ALSA plugin options (hw/plughw/pcm)

INTEGRATION:
- Registered commands in commands/mod.rs
- Added to invoke_handler in lib.rs
- All commands return pretty names for UI display

VERIFIED:
- cargo check passes successfully
- Ready for Phase 3 (settings persistence)

src-tauri/src/commands/mod.rs

@@ -2,6 +2,7 @@
 //!
 //! Exposes backend functionality to the frontend via IPC
 
+pub mod audio_backends;
 pub mod audio_diagnostics;
 pub mod auth;
 pub mod cache;
@@ -15,6 +16,7 @@ pub mod queue;
 pub mod search;
 pub mod share;
 
+pub use audio_backends::*;
 pub use audio_diagnostics::*;
 pub use auth::*;
 pub use cache::*;