refactor(audio): extract audio_preload into preload_commands

Pull audio_preload (background fetch + analysis seed for the next
track in the queue) out of audio/commands.rs into
audio/preload_commands.rs (~67 LOC). It's a self-contained
fetch-and-cache flow — distinct from audio_chain_preload (which
constructs the gapless source chain) and audio_play (which starts
playback) so it makes sense to live alongside them rather than inside
the same file.

audio/commands.rs: 1241 → 1185 LOC. lib.rs invoke_handler updated.

src-tauri/src/lib.rs

@@ -397,7 +397,7 @@ pub fn run() {
             audio::mix_commands::audio_set_eq,
             audio::autoeq_commands::autoeq_entries,
             audio::autoeq_commands::autoeq_fetch_profile,
-            audio::commands::audio_preload,
+            audio::preload_commands::audio_preload,
             audio::radio_commands::audio_play_radio,
             audio::preview::audio_preview_play,
             audio::preview::audio_preview_stop,