refactor(audio): peel device commands out of commands.rs

Hotspot J first slice: split the device-listing + device-selection
commands out of audio/commands.rs (1912 LOC monolith) into a new
audio/device_commands.rs (~95 LOC). Moved:
- audio_canonicalize_selected_device
- audio_list_devices_for_engine (kept pub for cli/exchange.rs callers)
- audio_list_devices
- audio_default_output_device_name
- audio_set_device

audio/mod.rs re-exports audio_default_output_device_name +
audio_list_devices_for_engine from the new submodule. The four
#[tauri::command] device handlers are registered in lib.rs run() under
their new path `audio::device_commands::*`.

audio/commands.rs goes from 1912 → 1830 LOC and drops its
`use super::dev_io::*` since playback/radio/EQ don't touch device
enumeration. Behaviour-preserving — same Tauri commands, same dev_io
helpers, same selected_device + stream_handle mutations.

src-tauri/src/lib.rs

@@ -406,10 +406,10 @@ pub fn run() {
             audio::commands::audio_set_crossfade,
             audio::commands::audio_set_gapless,
             audio::commands::audio_set_normalization,
-            audio::commands::audio_list_devices,
-            audio::commands::audio_canonicalize_selected_device,
-            audio::commands::audio_default_output_device_name,
-            audio::commands::audio_set_device,
+            audio::device_commands::audio_list_devices,
+            audio::device_commands::audio_canonicalize_selected_device,
+            audio::device_commands::audio_default_output_device_name,
+            audio::device_commands::audio_set_device,
             audio::commands::audio_chain_preload,
             discord::discord_update_presence,
             discord::discord_clear_presence,