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/components/PlayerBar.svelte

@@ -706,15 +706,17 @@
         border-radius: 2px;
     }
 
+    /* Always the accent — matching the waveform scrubber's played region
+       (user decision 2026-07-23; the old white-until-hover read as off). */
     .fill {
         position: relative;
         height: 100%;
-        background: var(--text-primary);
+        background: var(--accent);
         border-radius: 2px;
     }
 
     .track:hover .fill {
-        background: var(--accent);
+        background: var(--accent-hover);
     }
 
     /* waveform scrubber */