refactor(audio): split monolithic audio module into submodules

Move audio engine functionality from a single large file into focused Rust modules while preserving behavior and command surface. This keeps boundaries clearer for future extraction and the upcoming lib.rs split.

src-tauri/src/audio/mod.rs

@@ -0,0 +1,24 @@
+//! Audio playback: Symphonia decode, rodio output, HTTP radio/streaming, gapless, previews.
+//!
+//! Implementation is split into submodules (`sources`, `decode`, `stream`, `commands`, …)
+//! for navigation; behavior matches the historical single `audio.rs` file.
+
+mod codec;
+pub mod commands;
+mod decode;
+mod dev_io;
+mod device_watcher;
+mod engine;
+mod helpers;
+mod ipc;
+pub mod preview;
+mod sources;
+mod state;
+mod stream;
+
+pub use commands::{audio_default_output_device_name, audio_list_devices_for_engine};
+pub use device_watcher::start_device_watcher;
+pub use engine::{create_engine, refresh_http_user_agent, AudioEngine};
+pub use helpers::take_stream_completed_for_url;
+
+pub(crate) use engine::{analysis_track_id_is_current_playback, ranged_loudness_backfill_should_defer};