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/audio/commands.rs

@@ -1183,59 +1183,3 @@ pub(super) fn spawn_progress_task(
 
 
 
-#[tauri::command]
-pub async fn audio_preload(
-    url: String,
-    duration_hint: f64,
-    analysis_track_id: Option<String>,
-    app: AppHandle,
-    state: State<'_, AudioEngine>,
-) -> Result<(), String> {
-    {
-        let preloaded = state.preloaded.lock().unwrap();
-        if preloaded.as_ref().is_some_and(|p| same_playback_target(&p.url, &url)) {
-            let _ = app.emit("audio:preload-ready", url.clone());
-            return Ok(());
-        }
-    }
-    // Throttle: wait 8 s before starting the background download so it does not
-    // compete with the decode + sink-feed work of the just-started current track.
-    // If the user skips during the wait the generation counter changes and we abort.
-    let gen_snapshot = state.generation.load(Ordering::Relaxed);
-    tokio::time::sleep(Duration::from_secs(8)).await;
-    if state.generation.load(Ordering::Relaxed) != gen_snapshot {
-        return Ok(());
-    }
-    let data: Vec<u8> = if let Some(path) = url.strip_prefix("psysonic-local://") {
-        tokio::fs::read(path).await.map_err(|e| e.to_string())?
-    } else {
-        let response = audio_http_client(&state).get(&url).send().await.map_err(|e| e.to_string())?;
-        if !response.status().is_success() {
-            return Ok(());
-        }
-        response.bytes().await.map_err(|e| e.to_string())?.into()
-    };
-    let _ = duration_hint; // kept in API for compatibility
-    let logical_trim = analysis_track_id
-        .as_ref()
-        .map(|s| s.trim().to_string())
-        .filter(|s| !s.is_empty());
-    if let Some(track_id) = analysis_cache_track_id(logical_trim.as_deref(), &url) {
-        crate::app_deprintln!(
-            "[stream] audio_preload: bytes ready track_id={} size_mib={:.2} — invoking full-track analysis",
-            track_id,
-            data.len() as f64 / (1024.0 * 1024.0)
-        );
-        let high = crate::audio::engine::analysis_track_id_is_current_playback(&state, &track_id);
-        if let Err(e) = crate::submit_analysis_cpu_seed(app.clone(), track_id.clone(), data.clone(), high).await {
-            crate::app_eprintln!("[analysis] preload seed failed for {}: {}", track_id, e);
-        }
-    }
-    let url_for_emit = url.clone();
-    *state.preloaded.lock().unwrap() = Some(PreloadedTrack { url, data });
-    let _ = app.emit("audio:preload-ready", url_for_emit);
-    Ok(())
-}
-
-
-