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/preview.rs

@@ -34,9 +34,10 @@ const PREVIEW_POSITION_INTERVAL: Duration = Duration::from_millis(100);
 /// smooth — while each step is one cheap volume command to the sink.
 const FADE_TICK: Duration = Duration::from_millis(15);
 
-/// The preview voices' resting volume. The player crate's own default; the
-/// renderer never sets a preview volume today, so fades ramp to this unless
-/// a `volume` control changes the target.
+/// The preview voices' resting volume before the renderer speaks. Since
+/// 2026-07-23 the renderer mirrors the APP volume (mute → 0) via the
+/// `volume` control at session open and live while the modal is up, so
+/// this default only covers the instant before that first set arrives.
 const DEFAULT_TARGET_VOL: f32 = 0.7;
 
 /// Lease-lifetime constants, mirroring `src/lib/preview/lease.js` — the two
@@ -111,6 +112,12 @@ impl PreviewState {
     }
 }
 
+/// The preview voices' current resting volume — diagnostics readout, so the
+/// debug overlay can prove the renderer's app-volume mirror actually lands.
+pub fn target_volume(state: &PreviewState) -> f32 {
+    state.target()
+}
+
 /// How long a crossfade waits for the incoming voice to become audible
 /// before ramping anyway. Local-file decode start is tens of ms; hitting
 /// this means the clip is broken (its error event fails the item) and the