Add audio visualizer backend infrastructure

- Add spectrum-analyzer crate for FFT processing
- Create visualizer module with:
  - RingBuffer: lockless SPSC buffer for audio samples
  - FFT processor: dedicated thread for spectrum analysis
  - TappedSource: rodio Source wrapper for sample capture
  - VisualizerTap: shared state for audio thread integration
- Add Tauri commands: set_visualizer_enabled, is_visualizer_enabled
- Register visualizer in AppState and lib.rs

Backend compiles. Next: integrate with audio thread to capture samples.

src-tauri/src/commands/mod.rs

@@ -24,6 +24,7 @@ pub mod search;
 pub mod share;
 pub mod smart_playlists;
 pub mod remote_metadata;
+pub mod visualizer;
 
 pub use audio_backends::*;
 pub use audio_diagnostics::*;
@@ -47,3 +48,4 @@ pub use search::*;
 pub use share::*;
 pub use smart_playlists::*;
 pub use remote_metadata::*;
+pub use visualizer::*;