fix(preview): keep preview sink volume in sync with player slider (#498) (#502)

* fix(preview): keep preview sink volume in sync with player slider (#498)

The Rust preview sink had its volume set once at audio_preview_play and
then never updated. audio_set_volume only ramps the main sink, so slider
movements during a preview had zero effect on the preview level. With
the default loudness normalization (-4.5 dB pre-analysis attenuation)
applied at start, even a 100% slider gives 1.0 × 0.596 × MASTER_HEADROOM
≈ 53% — matching the user-visible "fixed at around 50%" symptom.

- Add audio_preview_set_volume Rust command that updates the preview
  sink if one is active (clamp + master headroom mirror the path used
  in audio_preview_play).
- Extract the preview-volume calculation in previewStore into
  computePreviewVolume() so startPreview and the new sync path share
  one formula (slider value, plus the LUFS pre-analysis attenuation
  the engine already applies to the main sink).
- Subscribe to playerStore at module level: when volume changes and a
  preview is active, push the recomputed value to Rust. Auth /
  normalization tweaks during preview are intentionally not synced —
  preview is short and that case is rare.

Reported by netherguy4.

* docs: changelog entry for PR #502

Logs the preview-volume-slider sync fix in v1.46.0 "## Fixed".

src-tauri/src/lib.rs

@@ -915,6 +915,7 @@ pub fn run() {
             audio::preview::audio_preview_play,
             audio::preview::audio_preview_stop,
             audio::preview::audio_preview_stop_silent,
+            audio::preview::audio_preview_set_volume,
             audio::commands::audio_set_crossfade,
             audio::commands::audio_set_gapless,
             audio::commands::audio_set_normalization,