fix: previews follow the app volume; volume bar fill always accent

The preview backend never received a volume — clips played at the crate
default (0.7) regardless of the slider, even muted. The session now sets
the preview volume from the app store (mute → 0) before the first load and
mirrors it live while the modal is open; the debug overlay's PREVIEW row
exposes the backend's resting volume as proof.

The player-bar volume (and fallback seek) fill is now always the accent
color — matching the waveform scrubber's played region — stepping up to
the hover shade on hover instead of white-until-hover.

Also syncs package-lock to the soulstream name + alpha version.

src-tauri/src/diagnostics.rs

@@ -79,6 +79,8 @@ pub struct DebugStats {
     pub normalization_enabled: bool,
     pub target_lufs: f64,
     pub loudness: Option<crate::loudness::CurrentTrack>,
+    /// The preview voices' resting volume (mirrors the app volume; 0 = muted).
+    pub preview_volume: Option<f32>,
 }
 
 #[tauri::command]
@@ -163,6 +165,9 @@ pub fn debug_stats(app: tauri::AppHandle, state: State<'_, DiagState>) -> DebugS
         normalization_enabled,
         target_lufs,
         loudness,
+        preview_volume: app
+            .try_state::<crate::preview::PreviewState>()
+            .map(|s| crate::preview::target_volume(&s)),
     }
 }