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/lib/preview/service.js

@@ -348,6 +348,13 @@ export function createPreviewService({
             if (value && state.phase === PHASE.PLAYING) void transport.pause();
         },
 
+        /** Mirror the APP's volume (and mute, as 0) onto the preview
+         * transport — previews must never play louder than the player bar
+         * says. No-op for transports without a volume control (fixture). */
+        setVolume(v) {
+            void transport.setVolume?.(v);
+        },
+
         async togglePlay() {
             if (!state) return;
             const sid = state.sessionId;