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/DebugOverlay.svelte

@@ -124,6 +124,7 @@
                         <b>PREVIEW</b>
                         {previewUi.snapshot?.phase ?? "—"} · gen {previewUi.snapshot?.requestGeneration ?? "—"} ·
                         idx {previewUi.snapshot?.activeManifestIndex ?? "—"}
+                        {#if stats.previewVolume != null}· vol {stats.previewVolume.toFixed(2)}{/if}
                     </div>
                 {/if}
             </div>